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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Feb. 28, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Trading Symbol	NTLA
Entity Registrant Name	INTELLIA THERAPEUTICS, INC.
Entity Central Index Key	0001652130
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		42,387,435
Entity Public Float			$ 235.3

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current Assets:
Cash and cash equivalents	$ 340,678	$ 273,064
Accounts receivable	10,471	6,454
Prepaid expenses and other current assets	3,681	1,788
Total current assets	354,830	281,306
Property and equipment, net	15,272	10,628
Other assets	6,133	7,035
Total Assets	376,235	298,969
Current Liabilities:
Accounts payable	2,172	4,652
Accrued expenses	7,999	5,900
Current portion of deferred revenue	21,188	20,178
Total current liabilities	31,359	30,730
Deferred revenue, net of current portion	44,111	58,109
Other long-term liabilities	168	293
Commitments and contingencies (Note 6)
Stockholders’ Equity:
Common stock, $0.0001 par value; 120,000,000 shares authorized; 42,384,623 shares issued and outstanding and 36,018,540 shares issued and outstanding, respectively	4	4
Additional paid-in capital	421,706	263,403
Accumulated deficit	(121,113)	(53,570)
Total stockholders’ equity	300,597	209,837
Total Liabilities and Stockholders’ Equity	$ 376,235	$ 298,969

Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Collaboration revenue	$ 26,117	$ 16,479	$ 6,044
Operating expenses:
Research and development	67,647	31,840	11,170
General and administrative	28,025	16,798	8,283
Total operating expenses	95,672	48,638	19,453
Operating loss	(69,555)	(32,159)	(13,409)
Interest income	2,012	525
Loss before income taxes	(67,543)	(31,634)	(13,409)
Income tax benefit			1,012
Net loss	$ (67,543)	$ (31,634)	$ (12,397)
Net loss per share, basic and diluted	$ (1.88)	$ (1.42)	$ (51.02)
Weighted average shares outstanding, basic and diluted	36,006	22,222	243

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Novartis [Member] Collaborative Arrangement [Member]	Common Units [Member]	Common Stock [Member]	Incentive [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Series A-1, Series A-2 and Junior Preferred [Member] Junior Preferred Units [Member]	Series A-1, Series A-2 and Junior Preferred [Member] Junior Preferred Units [Member] Novartis [Member] Collaborative Arrangement [Member]	Convertible Preferred Stock (Series B, Series A-2, Series A-1, Junior and Founder) [Member]	Class A-2 Preferred Units [Member]	Series B Preferred Stock [Member]
Beginning balance at Dec. 31, 2014	$ 7,566		$ 607		$ 50		$ (9,539)	$ 16,448
Beginning balance, shares at Dec. 31, 2014			2,298,000		1,558,498			19,348,694
Issuance of units/stock	1,984										$ 1,984
Issuance of units/stock, shares											1,333,333
Allocation from Novartis collaboration to carrying value of Preferred Shares	2,600	$ 2,644							$ 2,644
Tax provision associated with intraperiod tax allocation	(1,012)							$ (1,012)
Effect of Reorganization	(20,671)		$ (607)		$ (50)	$ 50		$ (20,064)		$ 20,671
Effect of Reorganization, shares			(2,298,000)	1,713,104	(1,558,498)			(20,682,027)		22,980,027
Issuance of Series B Preferred Shares, net of issuance costs												$ 67,263
Issuance of Series B Preferred Shares, net of issuance costs, shares												13,336,601
Equity-based compensation	685					685				$ 623
Equity-based compensation, shares				845,651
Net loss	(12,397)						(12,397)
Ending balance at Dec. 31, 2015										$ 88,557
Ending balance at Dec. 31, 2015	(21,201)					735	(21,936)
Ending balance, shares at Dec. 31, 2015				2,558,755
Ending balance, shares at Dec. 31, 2015										36,316,628
Conversion of convertible preferred stock	88,557			$ 3		88,554				$ (88,557)
Conversion of convertible preferred stock, shares				23,481,956						(36,316,628)
Issuance of units/stock	167,139			$ 1		167,138
Issuance of units/stock, shares				9,955,554
Exercise of stock options	2					2
Exercise of stock options, shares				257
Issuance of shares under employee stock purchase plan	259					259
Issuance of shares under employee stock purchase plan, shares				23,209
Equity-based compensation	6,715					6,715
Equity-based compensation, shares				(1,191)
Net loss	(31,634)						(31,634)
Ending balance at Dec. 31, 2016	$ 209,837			$ 4		263,403	(53,570)
Ending balance, shares at Dec. 31, 2016	36,018,540			36,018,540
Issuance of units/stock	$ 141,000					141,000
Issuance of units/stock, shares				6,250,000
Exercise of stock options	$ 1,156					1,156
Exercise of stock options, shares	141,759			141,759
Issuance of shares under employee stock purchase plan	$ 825					825
Issuance of shares under employee stock purchase plan, shares				64,786
Equity-based compensation	15,322					15,322
Equity-based compensation, shares				(90,462)
Net loss	(67,543)						(67,543)
Ending balance at Dec. 31, 2017	$ 300,597			$ 4		$ 421,706	$ (121,113)
Ending balance, shares at Dec. 31, 2017	42,384,623			42,384,623

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (67,543)	$ (31,634)	$ (12,397)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Depreciation and amortization	2,994	1,104	328
Loss on disposal of property and equipment	166	13	9
Equity-based compensation	15,322	6,715	1,308
Changes in operating assets and liabilities:
Accounts receivable	(4,017)	(5,454)	(1,012)
Prepaid expenses and other current assets	(1,893)	(979)	(525)
Accounts payable	(488)	1,784	335
Accrued expenses	2,394	3,050	805
Deferred revenue	(12,988)	67,975	9,312
Other assets	902	(6,435)	(76)
Other long-term liabilities	(125)	(30)	150
Net cash (used in) provided by operating activities	(65,276)	36,109	(1,763)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(10,091)	(6,165)	(2,554)
Net cash used in investing activities	(10,091)	(6,165)	(2,554)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from sale of Class A-1 and A-2 preferred units and Series B preferred stock			74,661
Payments to acquire in-process research and development		(600)	(1,100)
Payment of preferred unit and preferred stock issuance costs		(100)	(2,671)
Proceeds from common stock offering	141,000	170,507
Proceeds from options exercised	1,156	2
Issuance of shares through employee stock purchase plan	825	259
Payment of common stock offering costs		(2,764)	(602)
Net cash provided by financing activities	142,981	167,304	70,288
Net increase in cash and cash equivalents	67,614	197,248	65,971
Cash and cash equivalents, beginning of period	273,064	75,816	9,845
Cash and cash equivalents, end of period	340,678	273,064	75,816
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Purchases of property and equipment unpaid at period end	$ 805	3,090	219
Acquisition of in-process research and development unpaid at period end			600
Financing costs incurred but unpaid at period end			$ 970
Conversion of convertible preferred stock to common stock		$ 88,557

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The consolidated financial statements include the accounts of Intellia Therapeutics, Inc. and its wholly-owned, controlled subsidiary, Intellia Securities Corp. All intercompany balances and transactions have been eliminated in consolidation. The only item comprising comprehensive loss is net loss.

Use of Estimates

The preparation of the Company’s consolidated financial statements in accordance with accounting principles generally accepted in the United States of America (“U.S.”) requires management to make estimates, judgments and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities as of the date of the consolidated financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, revenue recognition and the valuation of common and incentive units for the periods prior to the Company’s stock becoming publicly traded. Estimates are periodically reviewed in light of changes in circumstances, facts and experiences. Actual results may differ materially from management’s estimates, judgments and assumptions.

Fair Value Measurements

The Company classifies fair value based measurements using a three-level hierarchy that prioritizes the inputs used to measure fair value. This hierarchy requires entities to maximize the use of observable inputs and minimize the use of unobservable inputs. The three levels of inputs used to measure fair value are as follows: Level 1, quoted market prices in active markets for identical assets or liabilities; Level 2, observable inputs other than quoted market prices included in Level 1, such as quoted market prices for markets that are not active or other inputs that are observable or can be corroborated by observable market data; and Level 3, unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities, including certain pricing models, discounted cash flow methodologies and similar techniques that use significant unobservable inputs.

The Company’s financial instruments as of December 31, 2017 and 2016 consisted primarily of cash and cash equivalents, accounts receivable and accounts payable. As of December 31, 2017 and 2016, the Company’s financial assets recognized at fair value consisted of the following:

	Fair Value as of December 31, 2017
	Total		Level 1		Level 2		Level 3
	(In thousands)
Cash equivalents	$	330,896	$	330,896	$	-	$	-
Total	$	330,896	$	330,896	$	-	$	-

	Fair Value as of December 31, 2016
	Total		Level 1		Level 2		Level 3
	(In thousands)
Cash equivalents	$	270,448	$	270,448	$	-	$	-
Total	$	270,448	$	270,448	$	-	$	-

The Company values its cash equivalents at quoted market prices in active markets. Other financial instruments, including accounts receivable and accounts payable, are carried at cost, which approximate fair value due to the short duration and term to maturity.

Cash Equivalents

The Company considers all highly liquid investments with maturities of three months or less when purchased to be cash equivalents. As of December 31, 2017 and 2016, cash equivalents consisted of interest-bearing money market accounts, commercial paper and U.S. treasury securities.

Concentrations of Credit Risk

The Company’s cash and cash equivalents may potentially be subject to concentrations of credit risk. The Company generally maintains balances in various accounts in excess of federally insured limits with financial institutions that management believes to be of high credit quality.

Accounts receivable represent amounts due from collaboration partners. The Company monitors economic conditions to identify facts or circumstances that may indicate that any of its accounts receivable are at risk of collection. As of December 31, 2017 and 2016, the Company’s two collaboration partners, Regeneron Pharmaceuticals, Inc. (“Regeneron”) and Novartis Institutes for BioMedical Research, Inc. (“Novartis”), accounted for all of the Company’s accounts receivable.

Property and Equipment

The Company records property and equipment at cost and recognizes depreciation and amortization using the straight-line method over the following estimated useful lives of the respective assets:

Asset Category		Useful Life
Laboratory equipment		5 years
Office furniture and equipment		5 years
Computer software		3 years
Computer equipment		3 years
Leasehold improvements		5 years or term of respective lease, if shorter

Expenditures for repairs and maintenance of assets are expensed as incurred. Upon retirement or sale, the cost of assets disposed and the corresponding accumulated depreciation are removed from the related accounts and any resulting gain or loss is reflected in the results of operations.

Impairment of Long-Lived Assets

The Company tests long-lived assets to be held and used, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying amount of assets or asset groups may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets.

Evaluation of recoverability of the asset or asset group is based on an estimate of undiscounted future cash flows resulting from the use of the asset or asset group and its eventual disposition. In the event that such cash flows are not expected to be sufficient to recover the carrying amount of the asset or asset group, the assets are written down to their estimated fair values. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows. To date, the Company has not recorded any material impairment losses on long-lived assets.

Income Taxes

The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences attributable to differences between carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax reporting purposes and for operating loss and tax credit carryforwards. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes.

The Company’s deferred tax assets and liabilities are measured using enacted tax rates expected to apply in the years in which these temporary differences are expected to be recovered or settled. A valuation allowance is recorded to reduce deferred tax assets if it is determined that it is more likely than not that all or a portion of the deferred tax asset will not be realized. The Company considers many factors when assessing the likelihood of future realization of deferred tax assets, including recent earnings results, expectations of future taxable income, carryforward periods available and other relevant factors. The Company records changes in the required valuation allowance in the period that the determination is made.

The Company assesses its income tax positions and records tax benefits for all years subject to examination based upon management’s evaluation of the facts, circumstances and information available as of the reporting date. For those tax positions where it is more likely than not that a tax benefit will be sustained, the Company records the largest amount of tax benefit with a greater than 50.0% likelihood of being realized upon ultimate settlement with a taxing authority having full knowledge of all relevant information. For those income tax positions where it is not more likely than not that a tax benefit will be sustained, the Company does not recognize a tax benefit in the financial statements. The Company records interest and penalties related to uncertain tax positions, if applicable, as a component of income tax expense.

Revenue Recognition

The Company recognizes revenue for each identified unit of accounting when all of the following criteria are met: persuasive evidence of an arrangement exists; delivery has occurred or services have been rendered; the seller’s price to the buyer is fixed or determinable; and collectability is reasonably assured.

The terms of the Company’s collaboration and license agreements contain multiple deliverables, which include licenses to CRISPR/Cas9-based therapeutic products directed to specific targets, referred to as exclusive licenses, as well as research and development activities to be performed by the Company on behalf of the collaboration partner related to the licensed targets. Payments that the Company may receive under these types of agreements include non-refundable technology access fees, payments for research activities, payments based upon the achievement of specified milestones and royalties on any resulting net product sales.

Multiple-Element Arrangements

The Company’s collaboration and license agreements represent multiple-element arrangements. The Company evaluates its collaborative agreements for proper classification in its statements of operations based on the nature of the underlying activity. The Company generally reflects as revenue amounts due under its collaborative agreements related to reimbursement of development activities as the Company is generally the principal under the arrangement.

The Company evaluates multiple-element arrangements to determine (i) the deliverables included in the arrangement and (ii) whether the individual deliverables represent separate units of accounting or whether they must be accounted for as a combined unit of accounting. When deliverables are separable, consideration received is allocated to the separate units of accounting based on the relative selling price method and the appropriate revenue recognition principles are applied to each unit. When the Company determines that an arrangement should be accounted for as a single unit of accounting, the Company must determine the period over which the various elements will be delivered and revenue will be recognized. This evaluation requires the Company to make judgments about the individual deliverables and whether such deliverables are separable from the other aspects of the contractual relationship. Deliverables are considered separate units of accounting provided that (i) the delivered item has value to the customer on a standalone basis and (ii) if the arrangement includes a general right of return with respect to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in the Company’s control. In assessing whether an item has standalone value, the Company considers factors such as the research, development, manufacturing and commercialization capabilities of the collaboration partner and the availability of the associated expertise in the general marketplace. In addition, the Company considers whether the collaboration partner can use any other deliverable for its intended purpose without the receipt of the remaining deliverable, whether the value of the deliverable is dependent on the undelivered item, and whether there are other vendors that can provide the undelivered items.

The consideration received under an arrangement that is fixed or determinable is then allocated among the separate units of accounting based on the relative selling prices. The Company determines the selling price of a unit of accounting within each arrangement using vendor-specific objective evidence of selling price, if available; third-party evidence of selling price if vendor-specific objective evidence is not available; or best estimate of selling price, if neither vendor-specific objective evidence nor third-party evidence is available. Determining the best estimate of selling price for a unit of accounting requires significant judgment. In developing the best estimate of selling price for a unit of accounting, the Company considers applicable market conditions and relevant entity-specific factors, including factors that were contemplated in negotiating the agreement with the customer and estimated costs. The Company validates the best estimate of selling price for units of accounting by evaluating whether changes in the key assumptions used to determine the best estimate of selling price will have a significant effect on the allocation of arrangement consideration between multiple units of accounting.

The Company recognizes arrangement consideration allocated to each unit of accounting when all of the revenue recognition criteria are satisfied for that particular unit of accounting. In the event that a deliverable does not represent a separate unit of accounting, the Company recognizes revenue from the combined unit of accounting over the contractual or estimated performance period for the undelivered items, which is typically the term of the Company’s research and development obligations. If there is no discernible pattern of performance or objectively measurable performance measures do not exist, then the Company recognizes revenue under the arrangement on a straight-line basis over the period the Company is expected to satisfy its performance obligations. Conversely, if the pattern of performance over which the service is provided to the customer can be determined and objectively measurable performance measures exist, then the Company recognizes revenue under the arrangement using the proportional performance method. Revenue recognized is limited to the lesser of the cumulative amount of payments received or the cumulative amount of revenue earned, as determined using the straight-line method or proportional performance method, as applicable, as of the period ending date.

Significant management judgment is required in determining the level of effort required under an arrangement and the period over which the Company is expected to complete its performance obligations under an arrangement. Steering committee services that are not inconsequential or perfunctory and that are determined to be performance obligations are combined with other research services or performance obligations required under an arrangement, if any, in determining the level of effort required in an arrangement and the period over which the Company expects to complete its aggregate performance obligations.

Milestone Revenue

The Company’s collaboration and license agreements include contingent milestone payments related to specific development, regulatory and sales-based milestones. Development and regulatory milestones are typically payable when a product candidate initiates or advances in clinical trial phases, upon submission for marketing approval with regulatory authorities, and upon receipt of actual marketing approvals for a therapeutic or for additional indications. Sales-based milestones are typically payable when annual sales reach specified levels.

The Company evaluates whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. This evaluation includes an assessment of whether: (i) the consideration is commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the delivered item as a result of a specific outcome resulting from the Company’s performance to achieve the milestone, (ii) the consideration relates solely to past performance, and (iii) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. The Company evaluates factors such as the scientific, clinical, regulatory, commercial and other risks that must be overcome to achieve the particular milestone and the level of effort and investment required to achieve the particular milestone in making this assessment. There is considerable judgment involved in determining whether a milestone satisfies all of the criteria required to conclude that a milestone is substantive. The Company will recognize revenue in its entirety upon successful accomplishment of any substantive milestones, assuming all other revenue recognition criteria are met. Milestones that are not considered substantive are recognized as earned if there are no remaining performance obligations or over the remaining period of performance, with a cumulative catch-up being recognized for the elapsed portion of the period of performance, assuming all other revenue recognition criteria are met.

Non-refundable research, development and regulatory milestones that are expected to be achieved as a result of the Company’s efforts during the period of its performance obligations under the collaboration and license agreements may be considered to be substantive and are recognized as revenue upon the achievement of the milestone, assuming all other revenue recognition criteria are met. If not considered to be substantive, revenue from achievement of milestones is initially deferred and recognized over the remaining term of its performance obligations. Milestones that are not considered substantive because the Company does not contribute effort to their achievement are recognized as revenue upon achievement, assuming all other revenue recognition criteria are met, as there are no undelivered elements remaining and no continuing performance obligations on the Company’s part.

Amounts received prior to satisfying the revenue recognition criteria listed above are recorded as deferred revenue in the accompanying balance sheets. Although the Company follows detailed guidelines in measuring revenue, certain judgments affect the application of the Company’s revenue policy. For example, in connection with its existing collaboration agreements, the Company has recorded on its balance sheet short-term and long-term deferred revenue based on its best estimate of when such revenue will be recognized. However, this estimate is based on the Company’s current research plan and, if its research plan should change in the future, the Company may recognize a different amount of deferred revenue over the following 12-month period.

The estimate of deferred revenue also reflects management’s estimate of the periods of the Company’s involvement in its collaborations. The Company’s primary performance obligations under these collaborations consist of research and development services. In certain instances, the timing of satisfying these obligations can be difficult to estimate. Accordingly, the Company’s estimates may change in the future. Such changes to estimates would result in a change in prospective revenue recognition amounts. If these estimates and judgments change over the course of any of the Company’s collaborative agreements, it may affect the timing and amount of revenue that the Company will recognize and record in future periods.

Research and Development Expenses

Research and development costs are expensed as incurred. Research and development expenses consist of salaries, equity-based compensation and benefits of employees, lab supplies and materials, facilities expenses, overhead expenses, fees paid to subcontractors and contract research organizations and other external expenses.

The Company records payments made for research and development services prior to the services being rendered as prepaid expense on the consolidated balance sheet and expenses them as the services are provided. Contracts for multi-year research and development services are recorded on a straight-line basis over each annual contractual period based on the total contractual fee when the services rendered are expected to be substantially equivalent over the term of the arrangement. The cost of obtaining licenses for certain technology or intellectual property is recorded to research and development expense when incurred if the licensed technology or intellectual property has not yet reached technological feasibility and has no alternative future use.

Equity-Based Compensation

The Company measures employee equity-based compensation based on the grant date fair value of the equity awards using the Black-Scholes option pricing model. Equity-based compensation expense is recognized on a straight-line basis over the requisite service period of the awards and is adjusted for pre-vesting forfeitures in the period in which the forfeitures occur. For equity awards that have a performance condition, the Company recognizes compensation expense based on its assessment of the probability that the performance condition will be achieved.

The Company measures equity awards granted to consultants and non-employees based on the fair value of the award on the date each portion of the award vests, which represents when the Company receives the related goods or services. Compensation expense is recognized over the period during which services are rendered by such consultants and non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is remeasured using the then-current fair value of that equity award.

The Company classifies equity-based compensation expense in its consolidated statement of operations in the same manner in which the award recipient’s salary and related costs are classified or in which the award recipient’s service payments are classified.

(Loss) Earnings per Share

The Company calculates basic (loss) earnings per share by dividing (loss) income allocable to common stockholders by the weighted average number of common shares outstanding. During periods of income, the Company allocates to participating securities a proportional share of income determined by dividing total weighted average participating securities by the sum of the total weighted average common shares and participating securities (the “two-class method”). The Company’s preferred stock and restricted common stock have rights to earnings and to participate in distributions of the Company and are therefore considered to be participating securities. Participating securities have the effect of diluting both basic and diluted earnings per share during periods of income. During periods of loss, the Company allocates no loss to preferred units or preferred stock because they have no contractual obligation to share in the losses of the Company. The Company computes diluted (loss) earnings per share after giving consideration to the dilutive effect of stock options that are outstanding during the period, except where such non-participating securities would be anti-dilutive.

Segment Information

The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s one business segment is the development of gene editing-based therapies. All of the Company’s assets are held in the U.S. and all of the Company’s revenue has been generated in the U.S.

Recent Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes existing revenue recognition guidance. The standard’s core principle is that a company will recognize revenue when it transfers promised goods or services to future, potential customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The standard defines a five-step process to achieve this principle and will require companies to use more judgment and make more estimates than under the current guidance. These judgments and estimates will include identifying performance obligations in the customer contract, estimating the amount of variable consideration to include in the transaction price, and allocating the transaction price to each separate performance obligation. ASU 2014-09 also requires additional disclosure about the nature, amount, timing, and uncertainty of revenue and cash flows arising from customer contracts. The Company’s collaboration agreements with Novartis and Regeneron, which are discussed further in Note 7, are its sole sources of revenue and the only arrangements affected by the adoption of the new standard.

The Company will adopt the new standard effective January 1, 2018 under the modified retrospective method. During the fourth quarter of 2017, the Company substantially completed its assessment of the impact that the new standard will have on its consolidated financial statements. The Company preliminarily expects that the most significant impact of adopting the new standard relates to the treatment of quarterly research payments due to the Company under the collaboration with Novartis, as discussed further in Note 7. Under current generally accepted accounting principles, or GAAP, the Company does not account for these payments until the period they are received as they are not considered determinable. Under the new standard, the Company will include an estimate of variable consideration related to the quarterly research payments in the transaction price at the inception of the arrangement. The Company preliminarily expects that this change would have resulted in the recognition of additional revenue of approximately $5.4 million through December 31, 2017, which will be reflected as a credit to accumulated deficit on January 1, 2018. The Company preliminarily expects that revenue recognition related to the collaboration with Regeneron, as discussed further in Note 7, will remain substantially unchanged.

In February 2016, the FASB issued ASU No. 2016-02, Leases. ASU 2016-02 amends ASC 840, Leases, by introducing a lessee model that requires balance sheet recognition of most leases. The Company is the lessee under certain leases that are accounted for as operating leases. The proposed changes would require that substantially all of the Company’s operating leases be recognized as assets and liabilities on the Company’s balance sheet. ASU 2016-02 will be effective for the Company for annual periods, and interim periods within those annual periods, beginning January 1, 2019. The Company is evaluating the impact that the adoption of ASU 2016-02 will have on its consolidated financial statements but expects that the Company will recognize a significant lease obligation upon adoption. See Note 6 for additional information related to the Company’s lease obligations.

Income Taxes

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Income Taxes

5. Income Taxes

The Company did not record net income tax benefits for the operating losses incurred during the periods presented due to the uncertainty of realizing a tax benefit from those losses. Accordingly, any benefit recorded related to these deferred tax assets was offset by a valuation allowance reflecting management’s conclusion that realization of those assets was not more likely than not.

Intraperiod tax allocation rules require the allocation of the provision for income taxes between continuing operations and other categories of earnings, such as items credited directly to members’ equity. In periods in which the Company has a year-to-date pre-tax loss from continuing operations and has pre-tax income in other categories of earnings, the Company must allocate the income tax provision to the other categories of earnings. The Company then records a related income tax benefit in continuing operations.

During the year ended December 31, 2015, the Company allocated $2.6 million from the $30.0 million total fixed amount of consideration under the collaboration agreement with Novartis to the carrying value of the Class A-1 and A-2 Preferred Units to record those units based on their fair value at date of issuance. As a result of this allocation, during the year ended December 31, 2015, the Company recorded an income tax provision of $1.0 million within members’ equity as well as a corresponding income tax benefit of $1.0 million within continuing operations. Refer to Note 7, Collaborations, for additional information regarding this difference in value.

A reconciliation of the federal statutory income tax rate and the Company’s effective income tax rate is as follows:

	Year Ended December 31,
	2017			2016			2015
Federal statutory income tax rate		(34.0	)%		(34.0	)%		(34.0	)%
State income taxes		(6.9	)		(6.7	)		(4.4	)
Intraperiod tax allocation		-			-			(6.7	)
Permanent items		-			-			2.2
Research and development tax credits		(3.4	)		(2.8	)		(1.8	)
Stock-based compensation		3.6			1.9			1.1
Change in U.S. tax rate		18.7			-			-
Change in valuation allowance		22.0			41.6			36.0
Effective income tax rate		-	%		-	%		(7.6	)%

The Company’s net deferred tax assets (liabilities) consisted of the following:

	December 31,
	2017			2016
	(in thousands)
Deferred tax assets:
Intangibles, including acquired in-process research and development	$	1,303		$	2,035
Capitalized start-up costs		502			785
Net operating loss carryforwards		9,406			13,751
Research and development credit carryforwards		5,817			1,873
Deferred revenue		15,913			1,193
Equity-based compensation		3,248			1,940
Accruals and allowances		1,112			1,139
Gross deferred tax assets		37,301			22,716
Deferred tax asset valuation allowance		(35,372	)		(20,549	)
Total deferred tax assets		1,929			2,167
Deferred tax liabilities:
Fixed assets		(1,929	)		(2,167	)
Total deferred tax liabilities		(1,929	)		(2,167	)
Net deferred tax asset (liability)	$	-		$	-

As of December 31, 2017, the Company had federal and state net operating loss carryforwards of $36.7 million and $27.8 million, respectively, which begin to expire in 2034. As of December 31, 2017, the Company had federal and state research and development tax credit carryforwards of approximately $3.4 million and $2.5 million, which begin to expire in 2034 and 2030, respectively.

The Company evaluated the expected realizability of its net deferred tax assets as of December 31, 2017 and 2016 and determined that there was significant negative evidence due to its net operating loss position and insufficient positive evidence to support the realizability of these net deferred tax assets. The Company concluded it is more likely than not that its net deferred tax assets would not be realized in the future; therefore, the Company has provided a full valuation allowance against its net deferred tax asset balance as of December 31, 2017 and 2016. The valuation allowance increased by $14.8 million in 2017, $13.1 million in 2016 and $3.8 million in 2015.

Utilization of the net operating loss and research and development credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986, as amended, due to ownership changes that have occurred previously or that could occur in the future. These ownership changes may limit the amount of net operating loss and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax expense, respectively. The Company has not yet conducted a study to assess whether a change of control, as defined in Section 382, has occurred or whether there have been multiple changes in control since inception.

As of December 31, 2017, the Company had not identified any unrecognized tax benefits. The Company files income tax returns in the U.S. federal tax jurisdiction and Massachusetts and various other state tax jurisdictions. The Company is subject to examination by the Internal Revenue Service and Massachusetts taxing authorities. The returns in these jurisdictions since inception remain open for examination; however, there are currently no pending tax examinations.

On December 22, 2017, the Tax Cuts and Jobs Act (“TCJA”) was enacted. This law substantially amended the Internal Revenue Code and among other things, permanently reduced the U.S. corporate income tax rate from 35% to 21%. On December 22, 2017, the SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”), which allows the recording of provisional amounts during a measurement period not to extend beyond one year of the enactment date. As a result of remeasuring the deferred tax assets and liabilities to the lower tax rate, the net deferred tax assets decreased by $12.6 million, which was offset by a decrease in the valuation allowance. In accordance with SAB 118, the amount the Company recorded is a provisional amount and a reasonable estimate at December 31, 2017. The final impact may differ from this provisional amount due to, among other things, changes in interpretations and assumptions the Company has made thus far and the issuance of additional regulatory or other guidance. The Company expects to complete the measurement of the final impact within the one year measurement period.

Collaborations	12 Months Ended
Collaborations	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborations	7. Collaborations Novartis Institutes for BioMedical Research In December 2014, the Company entered into a strategic collaboration agreement with Novartis primarily focused on the development of new ex vivo CRISPR/Cas9-based therapies using chimeric antigen receptor T cells (“CAR-T cells”) and hematopoietic stem cells (“HSCs”). Agreement Structure Under the terms of the collaboration, the Company and Novartis may research potential therapeutic, prophylactic and palliative ex vivo applications of the CRISPR/Cas9 technology in HSCs and CAR-T cells. The Company and Novartis agreed to conduct research of HSC targets under a research plan agreed upon by both parties. Within the HSC therapeutic space, Novartis may obtain exclusive rights to a limited number of these HSC targets, to be selected by Novartis in a series of selection windows, the last of which closes 90 days before the fifth anniversary of the effective date of the collaboration agreement. The Company has the right to choose a limited number of HSC targets for its exclusive development and commercialization per the specified selection schedule. Following these selections by Novartis and the Company, Novartis may obtain rights to research an additional limited number of HSC targets on a non-exclusive basis. If Novartis does not exercise its selection rights within each selection window, any such rights will be deemed forfeited by Novartis. Novartis is required to use commercially reasonable efforts to research, develop and commercialize a specified number of HSC products directed to each of their selected HSC targets. The Company also agreed to collaborate with Novartis on research activities for CAR-T cell targets pursuant to a CAR-T cell program research plan approved by the CAR-T cell subcommittee of the collaboration’s joint steering committee. After completion of the activities contemplated by the CAR-T cell program research plan, Novartis will assume sole responsibility for developing any products arising from that research plan and will be responsible for additional costs and expenses of developing, manufacturing and commercializing its selected research targets. Novartis is required to use commercially reasonable efforts to research, develop or commercialize at least one CAR-T cell product directed to at least one of its selected CAR-T cell targets. In the last two years of the five-year collaboration term, Novartis will have the option to select a limited number of targets for research, development and commercialization of in vivo therapies using the Company’s CRISPR/Cas9 platform, on a non-exclusive basis. Following Novartis’ selection of each in vivo target, Novartis may offer the Company the right to participate in the research and development of such targets, in which case an in vivo program research plan for such target will be entered into between the Company and Novartis. Novartis is required to use commercially reasonable efforts to research, develop or commercialize at least one in vivo product directed to each of its selected targets. Novartis’ in vivo target selections are subject to certain restrictions, including that the targets, or all targets within a limited number of organs: (i) have not already been reserved by the Company pursuant to our limited right to do so under the agreement; (ii) are not the subject of a collaboration or pending collaboration with a third party; and (iii) are not the subject of ongoing or planned research and development by the Company. The Company received an upfront technology access payment from Novartis of $10.0 million in January 2015 and is entitled to additional technology access fees of $20.0 million and quarterly research payments of $1.0 million, or up to $20.0 million in the aggregate, during the five-year research term. For each product under the collaboration, subject to certain conditions, the Company may be eligible to receive (i) up to $30.3 million in development milestones, including for the filing of an investigational new drug application and for the dosing of the first patient in each of Phase IIa, Phase IIb and Phase III clinical trials, (ii) up to $50.0 million in regulatory milestones for the product’s first indication, including regulatory approvals in the U.S. and European Union (“EU”), (iii) up to $50.0 million in regulatory milestones for the product’s second indication, if any, including U.S. and EU regulatory approvals, (iv) royalties on net sales in the mid-single digits, and (v) net sales milestone payments of up to $100.0 million. The Company may also be eligible to receive payments for: (i) each additional HSC target selected by Novartis beyond its initial defined allocation, (ii) each in vivo target that Novartis selects and (iii) any exercise by Novartis of certain license options under the agreement. Additionally, at the inception of the arrangement, Novartis invested $9.0 million to purchase the Company’s Class A-1 and Class A-2 Preferred Units. The difference between the cash proceeds received from Novartis for the units and the $11.6 million estimated fair value of those units at the date of issuance was determined to be $2.6 million. Accordingly, $2.6 million of the upfront technology access payment was allocated to record the preferred units purchased by Novartis at fair value. Collaboration Revenue Through December 31, 2017, excluding amounts allocated to Novartis’ purchase of the Company’s Class A-1 and Class A-2 Preferred Units, the Company had recorded a total of $34.4 million in cash and accounts receivable under the Novartis agreement. Through December 31, 2017, the Company has recognized $23.1 million of collaboration revenue, including $9.3 million in the year ended December 31, 2017, $7.8 million in the year ended December 31, 2016, and $6.0 million in year ended December 31, 2015, in the consolidated statements of operations related to this agreement. As of December 31, 2017 and 2016, the Company had accounts receivable of $6.0 million and $6.0 million, respectively, related to this agreement. As of December 31, 2017 and 2016, the Company had deferred revenue of $11.2 million and $11.6 million, respectively, related to this agreement. Regeneron Pharmaceuticals, Inc. In April 2016, the Company entered into a license and collaboration agreement with Regeneron Pharmaceuticals, Inc. (“Regeneron”). The agreement includes a product component to research, develop and commercialize CRISPR/Cas-based therapeutic products primarily focused on gene editing in the liver as well as a technology collaboration component, pursuant to which the Company and Regeneron will engage in research and development activities aimed at discovering and developing novel technologies and improvements to CRISPR/Cas technology to enhance the Company’s gene editing platform. Under this agreement, the Company also may access the Regeneron Genetics Center and proprietary mouse models to be provided by Regeneron for a limited number of the Company’s liver programs. Agreement Structure Under the terms of the collaboration, the Company and Regeneron have agreed to a target selection process, whereby Regeneron may obtain exclusive rights for up to 10 targets to be chosen by Regeneron during the collaboration term, subject to various adjustments and limitations set forth in the agreement. Of these 10 total targets, Regeneron may select up to five non-liver targets, while the remaining targets must be focused in the liver. At the inception of the agreement, Regeneron selected the first of its 10 targets, which will be subject to a co-development and co-commercialization arrangement between the Company and Regeneron. The Company retains the exclusive right to solely develop products for certain indications. During the target selection process, the Company has the right to choose additional liver targets for its own development using commercially reasonable efforts. Certain targets that either the Company or Regeneron select may be subject to further co-development and co-commercialization arrangements at the Company’s or Regeneron’s option, as applicable, which either can exercise pursuant to defined conditions. In addition, subject to certain restrictions, Regeneron will be able to replace a limited number of targets with substitute targets upon the payment of a specified replacement fee, in which case exclusive rights to the replaced target revert to the Company. Regeneron’s target selections are subject to certain additional restrictions, including that non-liver targets are not the subject of ongoing or planned research and development by the Company or are not the subject of a collaboration or pending collaboration with a third party. Research activities under the collaboration will be governed by evaluation and research and development plans that will outline the parties’ responsibilities under, anticipated timelines of and budgets for, the various programs. The Company will assist Regeneron with the preliminary evaluation of liver targets, and Regeneron will be responsible for preclinical research and the conduct of clinical development, manufacturing and commercialization of products directed to each of its exclusive targets under the oversight of a joint steering committee. The Company may assist, as requested by Regeneron, with the later discovery and research of product candidates directed to any selected target. For each selected target, Regeneron is required to use commercially reasonable efforts to submit regulatory filings necessary to achieve initial investigational new drug (“IND”) acceptance for at least one product directed to each applicable target, and following IND acceptance for at least one product, to develop and commercialize such product. In connection with this collaboration, Regeneron agreed to purchase $50.0 million of the Company’s common stock in a private placement concurrent with the Company’s initial public offering, and the Company received a nonrefundable upfront payment of $75.0 million. In addition, the Company is eligible to earn, on a per-licensed target basis, (i) up to $25.0 million in development milestones, including for the dosing of the first patient in each of Phase I, Phase II and Phase III clinical trials, (ii) up to $110.0 million in regulatory milestones, including for the acceptance of a regulatory filing in the U.S., and U.S. and ex-U.S. regulatory approvals, and (iii) up to $185.0 million in sales-based milestone payments. The Company is also eligible to earn royalties ranging from the high single digits to low teens, in each case, on a per-product basis, which royalties are potentially subject to various reductions and offsets and are further subject to the Company’s existing low single-digit royalty obligations under a license agreement with Caribou Biosciences, Inc. (“Caribou”). In addition, Regeneron is obligated to fund 50.0% of the research and development costs for the transthyretin amyloidosis program, the first target selected by Regeneron, which will be subject to a co-development and co-commercialization arrangement between the Company and Regeneron. The fixed portion of consideration under the collaboration arrangement was determined to be the $75.0 million nonrefundable upfront payment, for which there are no contingent terms. The significant deliverables of this multiple-element revenue arrangement were determined to be licenses to targets, the associated research activities and evaluation plans for these programs and the technology collaboration. The Company further determined that the licenses and associated research activities and evaluation plans did not have standalone value due to the specialized nature of the services to be provided by the Company; therefore, these deliverables are not separable, and, accordingly, the license and services are treated as a single unit of accounting. The Company additionally concluded that the technology collaboration has standalone value from the product development, as shared rights to technological advancements under the technology collaboration could be separately applied by Regeneron to other programs. The Company allocated the $75.0 million in fixed consideration to the two units of accounting based on the estimated relative selling price of each deliverable. The Company estimated the selling price of each deliverable by taking into consideration internal estimates of research and development personnel needed to perform the research and development services, estimates of expected cash outflows to third parties for services and supplies, selling prices of comparable transactions and typical gross profit margins. As a result of this evaluation, the Company allocated $63.8 million to the licenses to targets and the associated research activities and evaluation plans and $11.2 million to the technology collaboration. The $63.8 million allocated to the licenses to targets and the associated research activities and evaluation plans for these programs is being recognized over the six-year performance period of the arrangement. The $11.2 million allocated to the technology collaboration is being recognized over a period beginning with the inception of the technology collaboration in September 2016, through the end of the arrangement. Collaboration Revenue Through December 31, 2017, the Company recorded a $75.0 million upfront payment and $4.6 million for research and development services under the Regeneron agreement. The Company recognized $16.8 million and $8.7 million of collaboration revenue in the years ended December 31, 2017 and 2016, respectively, in the consolidated statements of operations. As of December 31, 2017 and 2016, the Company had deferred revenue of $54.1 million and $66.7 million, respectively, and accounts receivable of $4.5 million and $0.5 million, respectively, related to this agreement. Agreement Termination Rights The collaboration term ends in April 2022, except that Regeneron may make a one-time payment of $25.0 million to extend the term for an additional two-year period. The agreement will continue until the date when no royalty or other payment obligations are due, unless earlier terminated in accordance with the terms of the agreement. Regeneron’s royalty payment obligations expire on a country-by-country and product-by-product basis upon the later of (i) the expiration of the last valid claim of the royalty-bearing patents covering such product in such country, (ii) 12 years from the first commercial sale of such product in such country, or (iii) the expiration of regulatory exclusivity for such product. The Company may terminate the agreement on a target-by-target basis if Regeneron or any of its affiliates institutes a patent challenge against the Company’s CRISPR/Cas or certain other background patent rights. The Company may also terminate the agreement on a target-by-target basis if Regeneron does not proceed with the development of a product directed to a selected target within specified periods of time. Regeneron may terminate the agreement, without cause, upon 180 days written notice to the Company, either in its entirety or on a target-by-target basis, in which event, certain rights in the terminated targets and associated intellectual property revert to the Company, as described in the agreement. Following such termination, the Company may owe Regeneron royalties, in certain circumstances, up to mid-single digits on any terminated targets that the Company subsequently commercializes on a product-by-product basis for a period of 12 years after the first commercial sale of any such products. Either party may terminate the agreement either in its entirety or with respect to the technology collaboration or one or more of the targets selected by Regeneron, in the event of the other party’s uncured material breach.

Equity-Based Compensation

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Equity-Based Compensation

8. Equity-Based Compensation

Equity-based compensation expense is classified in the consolidated statements of operations as follows:

	Year Ended December 31,
	2017		2016		2015
	(In thousands)
Research and development	$	7,280	$	4,083	$	1,061
General and administrative		8,042		2,632		247
Total	$	15,322	$	6,715	$	1,308

Restricted Stock

Restricted stock is measured at the fair value of the underlying security. Prior to the IPO, the Company valued these awards by taking into consideration its most recently available valuation performed by management and the board of directors, considering the most recently available third-party valuations of the Company’s securities as well as additional qualitative factors. In the periods subsequent to the IPO, fair value was determined based on the quoted price of the Company’s common stock.

The following table summarizes the Company’s restricted stock activity, including converted Founder Stock, for the year ended December 31, 2017:

	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted stock as of December 31, 2016		1,361,855			$	0.81
Vested		(791,571	)			0.69
Forfeited		(90,462	)			1.34
Unvested restricted stock as of December 31, 2017		479,822			$	0.90

As of December 31, 2017, there was $2.5 million of unrecognized equity-based compensation expense related to restricted stock that is expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 1.0 years.

Stock Options

The weighted average grant date fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $12.43 per option for options granted during the year ended December 31, 2017, $6.48 per option for options granted during the year ended December 31, 2016 and $4.24 per option for options granted during the year ended December 31, 2015. Key assumptions used to apply this pricing model were as follows:

	Year Ended December 31,
	2017			2016			2015
Risk-free interest rate		2.0	%		1.3	%		1.5	%
Expected life of options	6.0 years			6.0 years			6.0 years
Expected volatility of underlying stock		93.9	%		88.0	%		82.6	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

The following is a summary of stock option activity for the year ended December 31, 2017:

	Number of Options			Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(In years)		(In thousands)
Outstanding at December 31, 2016		3,040,214		$	8.35
Granted		2,691,157			16.26
Exercised		(141,759	)		8.16
Forfeited		(884,164	)		12.55
Outstanding at December 31, 2017		4,705,448			12.09		7.63		35,087
Exercisable at December 31, 2017		1,685,074			9.06		5.26		17,172

As of December 31, 2017, there was $27.2 million of unrecognized compensation cost related to stock options that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 3.1 years.

Unaudited Quarterly Results

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Unaudited Quarterly Results

12. Unaudited Quarterly Results

The results of operations on a quarterly basis for the years ended December 31, 2017 and 2016 are set forth below:

	March 31, 2017			June 30, 2017			September 30, 2017			December 31, 2017
	(Amounts in thousands except per share data)
Collaboration revenue	$	6,215		$	5,917		$	7,317		$	6,668

Operating expenses:
Research and development		13,431			15,565			17,481			21,170
General and administrative		5,732			6,369			5,711			10,213
Total operating expenses		19,163			21,934			23,192			31,383

Operating loss		(12,948	)		(16,017	)		(15,875	)		(24,715	)

Interest income		317			424			519			752
Net loss	$	(12,631	)	$	(15,593	)	$	(15,356	)	$	(23,963	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.36	)	$	(0.45	)	$	(0.44	)	$	(0.61	)
Weighted average shares outstanding, basic and diluted		34,723			34,916			35,189			39,155

	March 31, 2016			June 30, 2016			September 30, 2016			December 31, 2016
	(Amounts in thousands except per share data)
Collaboration revenue	$	1,777		$	4,206		$	4,869		$	5,627

Operating expenses:
Research and development		5,225			7,423			7,861			11,331
General and administrative		3,246			3,729			4,705			5,118
Total operating expenses		8,471			11,152			12,566			16,449

Operating loss		(6,694	)		(6,946	)		(7,697	)		(10,822	)

Interest income		5			46			215			259
Net loss	$	(6,689	)	$	(6,900	)	$	(7,482	)	$	(10,563	)
Net loss per share attributable to common stockholders, basic and diluted	$	(9.89	)	$	(0.36	)	$	(0.22	)	$	(0.31	)
Weighted average shares outstanding, basic and diluted		676			19,121			34,316			34,507

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates

Fair Value Measurements

	Fair Value as of December 31, 2017
	Total		Level 1		Level 2		Level 3
	(In thousands)
Cash equivalents	$	330,896	$	330,896	$	-	$	-
Total	$	330,896	$	330,896	$	-	$	-

	Fair Value as of December 31, 2016
	Total		Level 1		Level 2		Level 3
	(In thousands)
Cash equivalents	$	270,448	$	270,448	$	-	$	-
Total	$	270,448	$	270,448	$	-	$	-

Cash Equivalents

Concentrations of Credit Risk

Property and Equipment

The Company records property and equipment at cost and recognizes depreciation and amortization using the straight-line method over the following estimated useful lives of the respective assets:

Asset Category		Useful Life
Laboratory equipment		5 years
Office furniture and equipment		5 years
Computer software		3 years
Computer equipment		3 years
Leasehold improvements		5 years or term of respective lease, if shorter

Impairment of Long-Lived Assets

Income Taxes

Revenue Recognition

Multiple-Element Arrangements

Milestone Revenue

Research and Development Expenses

Equity-Based Compensation

(Loss) Earnings per Share

Segment Information

Recent Accounting Pronouncements

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Schedule of Reconciliation of the Federal Statutory Income Tax Rate and the Company's Effective Income Tax Rate

A reconciliation of the federal statutory income tax rate and the Company’s effective income tax rate is as follows:

	Year Ended December 31,
	2017			2016			2015
Federal statutory income tax rate		(34.0	)%		(34.0	)%		(34.0	)%
State income taxes		(6.9	)		(6.7	)		(4.4	)
Intraperiod tax allocation		-			-			(6.7	)
Permanent items		-			-			2.2
Research and development tax credits		(3.4	)		(2.8	)		(1.8	)
Stock-based compensation		3.6			1.9			1.1
Change in U.S. tax rate		18.7			-			-
Change in valuation allowance		22.0			41.6			36.0
Effective income tax rate		-	%		-	%		(7.6	)%

Summary of Company's Net Deferred Tax Assets (Liabilities)

The Company’s net deferred tax assets (liabilities) consisted of the following:

	December 31,
	2017			2016
	(in thousands)
Deferred tax assets:
Intangibles, including acquired in-process research and development	$	1,303		$	2,035
Capitalized start-up costs		502			785
Net operating loss carryforwards		9,406			13,751
Research and development credit carryforwards		5,817			1,873
Deferred revenue		15,913			1,193
Equity-based compensation		3,248			1,940
Accruals and allowances		1,112			1,139
Gross deferred tax assets		37,301			22,716
Deferred tax asset valuation allowance		(35,372	)		(20,549	)
Total deferred tax assets		1,929			2,167
Deferred tax liabilities:
Fixed assets		(1,929	)		(2,167	)
Total deferred tax liabilities		(1,929	)		(2,167	)
Net deferred tax asset (liability)	$	-		$	-

Equity-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Equity-Based Compensation Expense

Equity-based compensation expense is classified in the consolidated statements of operations as follows:

	Year Ended December 31,
	2017		2016		2015
	(In thousands)
Research and development	$	7,280	$	4,083	$	1,061
General and administrative		8,042		2,632		247
Total	$	15,322	$	6,715	$	1,308

Summary of Restricted Stock Activity, Including Converted Founder Stock

The following table summarizes the Company’s restricted stock activity, including converted Founder Stock, for the year ended December 31, 2017:

	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted stock as of December 31, 2016		1,361,855			$	0.81
Vested		(791,571	)			0.69
Forfeited		(90,462	)			1.34
Unvested restricted stock as of December 31, 2017		479,822			$	0.90

Summary of Weighted Average Assumptions Used to Compute Fair Value of Option Granted

Key assumptions used to apply this pricing model were as follows:

	Year Ended December 31,
	2017			2016			2015
Risk-free interest rate		2.0	%		1.3	%		1.5	%
Expected life of options	6.0 years			6.0 years			6.0 years
Expected volatility of underlying stock		93.9	%		88.0	%		82.6	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

Summary of Stock Option Activity

The following is a summary of stock option activity for the year ended December 31, 2017:

	Number of Options			Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(In years)		(In thousands)
Outstanding at December 31, 2016		3,040,214		$	8.35
Granted		2,691,157			16.26
Exercised		(141,759	)		8.16
Forfeited		(884,164	)		12.55
Outstanding at December 31, 2017		4,705,448			12.09		7.63		35,087
Exercisable at December 31, 2017		1,685,074			9.06		5.26		17,172

Unaudited Quarterly Results (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Schedule of Results of Operations on Quarterly Basis

The results of operations on a quarterly basis for the years ended December 31, 2017 and 2016 are set forth below:

	March 31, 2017			June 30, 2017			September 30, 2017			December 31, 2017
	(Amounts in thousands except per share data)
Collaboration revenue	$	6,215		$	5,917		$	7,317		$	6,668

Operating expenses:
Research and development		13,431			15,565			17,481			21,170
General and administrative		5,732			6,369			5,711			10,213
Total operating expenses		19,163			21,934			23,192			31,383

Operating loss		(12,948	)		(16,017	)		(15,875	)		(24,715	)

Interest income		317			424			519			752
Net loss	$	(12,631	)	$	(15,593	)	$	(15,356	)	$	(23,963	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.36	)	$	(0.45	)	$	(0.44	)	$	(0.61	)
Weighted average shares outstanding, basic and diluted		34,723			34,916			35,189			39,155

	March 31, 2016			June 30, 2016			September 30, 2016			December 31, 2016
	(Amounts in thousands except per share data)
Collaboration revenue	$	1,777		$	4,206		$	4,869		$	5,627

Operating expenses:
Research and development		5,225			7,423			7,861			11,331
General and administrative		3,246			3,729			4,705			5,118
Total operating expenses		8,471			11,152			12,566			16,449

Operating loss		(6,694	)		(6,946	)		(7,697	)		(10,822	)

Interest income		5			46			215			259
Net loss	$	(6,689	)	$	(6,900	)	$	(7,482	)	$	(10,563	)
Net loss per share attributable to common stockholders, basic and diluted	$	(9.89	)	$	(0.36	)	$	(0.22	)	$	(0.31	)
Weighted average shares outstanding, basic and diluted		676			19,121			34,316			34,507

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Benefit [Line Items]
Consideration received under collaboration agreement				$ 2,600,000
Income tax provision within equity				1,000,000
Income tax provision				(1,012,000)
Net operating loss carryforwards, federal		$ 36,700,000
Net operating loss carryforwards, state		$ 27,800,000
Net operating loss carryforwards, expiration year		2034
Net operating loss carryforwards, expiration date description		Begin to expire in 2034
Research and development tax credit		$ 5,817,000	$ 1,873,000
Research and development tax credits, expiration date description		Begin to expire in 2034 and 2029
Increase in valuation allowance		$ 14,800,000	$ 13,100,000	$ 3,800,000
Unrecognized tax benefits		$ 0
U.S. corporate income tax rate		34.00%	34.00%	34.00%
Provisional amount for decrease in net deferred tax assets offset by decrease in valuation allowance due to remeasurement		$ 12,600,000
Maximum [Member]
Income Tax Benefit [Line Items]
Measurement period		1 year
Scenario, Plan [Member]
Income Tax Benefit [Line Items]
U.S. corporate income tax rate	21.00%
Federal [Member]
Income Tax Benefit [Line Items]
Research and development tax credit		$ 3,400,000
Research and development tax credits, expiration year		2034
State [Member]
Income Tax Benefit [Line Items]
Research and development tax credit		$ 2,500,000
Research and development tax credits, expiration year		2030
Novartis [Member]
Income Tax Benefit [Line Items]
Total fixed amount of consideration under collaboration agreement				$ 30,000,000

Income Taxes - Summary of Company's Net Deferred Tax Assets (Liabilities) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Intangibles, including acquired in-process research and development	$ 1,303	$ 2,035
Capitalized start-up costs	502	785
Net operating loss carryforwards	9,406	13,751
Research and development credit carryforwards	5,817	1,873
Deferred revenue	15,913	1,193
Equity-based compensation	3,248	1,940
Accruals and allowances	1,112	1,139
Gross deferred tax assets	37,301	22,716
Deferred tax asset valuation allowance	(35,372)	(20,549)
Total deferred tax assets	1,929	2,167
Deferred tax liabilities:
Fixed assets	(1,929)	(2,167)
Total deferred tax liabilities	$ (1,929)	$ (2,167)

Collaborations - Additional Information (Detail)	1 Months Ended		3 Months Ended								12 Months Ended			21 Months Ended	37 Months Ended
Collaborations - Additional Information (Detail)	Apr. 30, 2016 USD ($) Unit	Jan. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Difference between cash proceeds received and estimated fair value of preferred units													$ 2,600,000
Collaboration revenue			$ 6,668,000	$ 7,317,000	$ 5,917,000	$ 6,215,000	$ 5,627,000	$ 4,869,000	$ 4,206,000	$ 1,777,000	$ 26,117,000	$ 16,479,000	6,044,000
Novartis [Member] \| Class A Preferred Units [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from issuance of preferred stock		$ 9,000,000
Estimated fair value of units		11,600,000
Difference between cash proceeds received and estimated fair value of preferred units		2,600,000
Novartis [Member] \| Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development based milestone payments under agreement		30,300,000
Regulatory based milestone payments for first indication		50,000,000
Regulatory based milestone payments for second indication		50,000,000
Sales based milestone payments under agreement		100,000,000
Novartis [Member] \| Collaborative Arrangement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred Revenue Additions		10,000,000													$ 34,400,000
Technology access fees		20,000,000
Quarterly research payments		$ 1,000,000
Research term		5 years
Collaboration revenue											9,300,000	7,800,000	$ 6,000,000		23,100,000
Accounts receivable			6,000,000				6,000,000				6,000,000	6,000,000		$ 6,000,000	6,000,000
Deferred revenue			11,200,000				11,600,000				11,200,000	11,600,000		11,200,000	11,200,000
Novartis [Member] \| Collaborative Arrangement [Member] \| Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total research funding		$ 20,000,000
Regeneron Pharmaceuticals Inc. [Member] \| Collaborative Arrangement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deferred Revenue Additions	$ 75,000,000													75,000,000
Collaboration revenue											16,800,000	8,700,000
Deferred revenue			54,100,000				66,700,000				$ 54,100,000	66,700,000		54,100,000	54,100,000
Purchase of common stock through private placement	$ 50,000,000
Percentage of obligated fund on research and development costs	50.00%
Revenue Recognition, Multiple-deliverable Arrangements, Allocation to Specific Unit of Accounting											The Company allocated the $75.0 million in fixed consideration to the two units of accounting based on the estimated relative selling price of each deliverable.
Number of accounting based units \| Unit	2
Amount allocated to licenses to targets and associated research activities and evaluation plans	$ 63,800,000
Amount allocated to technology collaboration	$ 11,200,000
Licenses to targets and associated research activities and evaluation plans performance period	6 years
Accounts receivable			$ 4,500,000				$ 500,000				$ 4,500,000	$ 500,000		4,500,000	$ 4,500,000
One time collaboration payment	$ 25,000,000
Collaboration term extension period	2 years
Royalty payment obligation expiration period	12 years
Termination period of agreement	180 days
Regeneron Pharmaceuticals Inc. [Member] \| Collaborative Arrangement [Member] \| Research and Development Services [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration revenue														$ 4,600,000
Regeneron Pharmaceuticals Inc. [Member] \| Collaborative Arrangement [Member] \| Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Development based milestone payments under agreement	$ 25,000,000
Sales based milestone payments under agreement	185,000,000
Regulatory based milestone payments under agreement	$ 110,000,000

Equity-Based Compensation - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options, Outstanding, Beginning Balance \| shares	3,040,214
Number of options, Granted \| shares	2,691,157
Number of options, Exercised \| shares	(141,759)
Number of options, Forfeited \| shares	(884,164)
Number of Options, Outstanding, Ending Balance \| shares	4,705,448
Number of Options, Exercisable \| shares	1,685,074
Weighted Average Exercise Price per Share, Outstanding, Beginning Balance \| $ / shares	$ 8.35
Weighted Average Exercise Price per Share, Granted \| $ / shares	16.26
Weighted Average Exercise Price per Share, Exercised \| $ / shares	8.16
Weighted Average Exercise Price per Share, Forfeited \| $ / shares	12.55
Weighted Average Exercise Price per Share, Outstanding, Ending Balance \| $ / shares	12.09
Weighted Average Exercise Price per Share, Exercisable \| $ / shares	$ 9.06
Weighted Average Remaining Contractual Term, Outstanding	7 years 7 months 17 days
Weighted Average Remaining Contractual Term, Exercisable	5 years 3 months 3 days
Aggregate Intrinsic Value, Outstanding \| $	$ 35,087
Aggregate Intrinsic Value, Exercisable \| $	$ 17,172

Stockholders' Equity - Additional Information (Detail)					1 Months Ended	12 Months Ended
Stockholders' Equity - Additional Information (Detail)	Nov. 06, 2017 USD ($)	Nov. 01, 2017 $ / shares shares	May 11, 2016 USD ($) $ / shares shares	Apr. 25, 2016	May 31, 2016 shares	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($)
Class Of Stock [Line Items]
Number of common stock issued upon conversion of value						$ 141,000,000	$ 167,139,000	$ 1,984,000
Common stock par value per share \| $ / shares						$ 0.0001	$ 0.0001
Proceeds from common stock offering						$ 141,000,000	$ 170,507,000
Accretion amount recognized						$ 0
IPO [Member]
Class Of Stock [Line Items]
Number of common stock issued upon conversion of shares \| shares			6,900,000		6,900,000
Common stock price per share \| $ / shares			$ 18.00
Proceeds from issuance initial public offering			$ 115,500,000
Reverse stock split ratio of common stock				0.588
Reverse stock split description						one-for-1.7 reverse stock split
Effective date of reverse stock split						Apr. 25, 2016
Convertible preferred stock conversion ratio						one-for-0.6465903
Conversion of convertible preferred stock into common stock \| shares			23,481,956		23,481,956
Private Placement [Member]
Class Of Stock [Line Items]
Number of common stock issued upon conversion of shares \| shares			3,055,554		3,055,554
Number of common stock issued upon conversion of value			$ 55,000,000
Public Offering [Member] \| Underwriting Agreement [Member]
Class Of Stock [Line Items]
Number of common stock issued upon conversion of shares \| shares		6,250,000
Common stock par value per share \| $ / shares		$ 0.0001
Proceeds from common stock offering	$ 141,000,000

Related Party Transactions - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended								12 Months Ended
	May 31, 2016	Jul. 31, 2014	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Aug. 31, 2015
Related Party Transaction [Line Items]
Research and development expense related to license and service agreements with related party			$ 21,170	$ 17,481	$ 15,565	$ 13,431	$ 11,331	$ 7,861	$ 7,423	$ 5,225	$ 67,647	$ 31,840	$ 11,170
General and administrative expenses			10,213	5,711	6,369	5,732	5,118	4,705	3,729	3,246	28,025	16,798	8,283
Collaboration revenue			$ 6,668	$ 7,317	$ 5,917	$ 6,215	5,627	$ 4,869	$ 4,206	$ 1,777	$ 26,117	$ 16,479	$ 6,044
Class A-2 Preferred Units [Member]
Related Party Transaction [Line Items]
Common stock, shares acquired													1,333,333
Caribou [Member]
Related Party Transaction [Line Items]
Junior preferred units issued		8,110,599
Percentage of related party obligation		30.00%									30.00%	30.00%	30.00%
Percentage of shares owned			10.70%								10.70%
Research and development expense related to license and service agreements with related party												$ 1,300	$ 1,500
Current obligations related to license and service agreements with related party													600
General and administrative expenses											$ 500	900	1,100
Novartis [Member] \| Collaborative Arrangement [Member]
Related Party Transaction [Line Items]
Percentage of shares owned			9.70%								9.70%
Common stock, shares acquired	277,777
Collaboration revenue											$ 9,300	7,800	$ 6,000
Accounts receivable			$ 6,000				6,000				6,000	6,000
Deferred revenue			$ 11,200				$ 11,600				$ 11,200	$ 11,600
Novartis [Member] \| Class A-1 Preferred Units [Member] \| Collaborative Arrangement [Member]
Related Party Transaction [Line Items]
Junior preferred units issued		4,761,905
Novartis [Member] \| Class A-2 Preferred Units [Member] \| Collaborative Arrangement [Member]
Related Party Transaction [Line Items]
Junior preferred units issued		2,666,666
Novartis [Member] \| Series B Preferred Stock [Member] \| Collaborative Arrangement [Member]
Related Party Transaction [Line Items]
Preferred stock, shares acquired														761,905

Stockholders’ Equity	12 Months Ended
Stockholders’ Equity	Dec. 31, 2017
Equity [Abstract]
Stockholders’ Equity	10. Stockholders’ Equity On May 11, 2016, the Company completed an initial public offering (“IPO”) of its common stock, which resulted in the sale of 6,900,000 shares, including all additional shares available to cover over-allotments, at a price of $18.00 per share. The Company received net proceeds before expenses from the IPO of $115.5 million after deducting underwriting discounts and commissions paid by the Company. In preparation for the IPO, the Company’s board of directors and stockholders approved a one-for-1.7 reverse stock split of the Company’s common stock effective April 25, 2016. All share and per share amounts in the consolidated financial statements and notes thereto have been retroactively adjusted, where necessary, to give effect to this reverse stock split. In connection with the closing of the IPO, all of the Company’s outstanding convertible preferred stock automatically converted to common stock at a one-for-0.6465903 ratio as of May 11, 2016, resulting in an additional 23,481,956 shares of common stock of the Company becoming outstanding. In addition, the Company issued a total of 3,055,554 shares of common stock for $55.0 million in two separate, concurrent private placements upon the closing of the IPO. On November 1, 2017, the Company entered into an underwriting agreement related to a public offering of 6,250,000 shares of the Company’s common stock, par value $0.0001 per share. The offering closed on November 6th and the Company received net proceeds of $141.0 million, after deducting underwriting discounts. Reorganization In July 2014, Intellia Therapeutics, LLC was formed as the parent company of Intellia Therapeutics, Inc. In August 2015, the Company completed a series of transactions pursuant to which Intellia Therapeutics, LLC merged with and into its C corporation subsidiary, Intellia Therapeutics, Inc., with Intellia Therapeutics, Inc. continuing to exist as the surviving corporation (the “Reorganization”). In connection with the Reorganization, all of the outstanding common and preferred unitholders of Intellia Therapeutics, LLC received shares of preferred stock of Intellia Therapeutics, Inc., and holders of incentive units in Intellia Therapeutics, LLC received shares of restricted common stock in Intellia Therapeutics, Inc. The Company determined that the Reorganization lacked economic substance and should be accounted for in a manner consistent with a common control transaction. Similarly, as there was no change in fair value between stockholders, individually or as a class, the Company determined that the exchange of shares occurring in the Reorganization should be accounted for as a modification of the equity securities and presented as a reclassification of the components of equity. The Company classifies stock that is redeemable in circumstances outside of the Company’s control outside of permanent equity. The Company recorded convertible preferred stock at fair value upon issuance, net of any issuance costs or discounts. No accretion was recognized as the contingent events that could give rise to redemption were not deemed probable. The preferred stock outstanding at December 31, 2015 was automatically converted to common stock in connection with the closing of the IPO.

Related Party Transactions	12 Months Ended
Related Party Transactions	Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	11. Related Party Transactions Caribou Therapeutics In July 2014, the Company issued Caribou Therapeutics Holdco, LLC, a wholly-owned subsidiary of Caribou, 8,110,599 Junior Preferred Units and concurrently licensed certain intellectual property and entered into an arrangement under which Caribou provided research and development services. In addition, under the license agreement the Company agreed to pay 30% of Caribou’s patent prosecution, filing and maintenance costs under its intellectual property license agreement with Caribou. As a result of this and subsequent transactions, Caribou owned 10.7% of the Company’s voting interests as of December 31, 2017. During the year ended December 31, 2015, the Company recognized $1.5 million in research and development expense and, as of December 31, 2015, had recorded current obligations of $0.6 million related to the license and service agreements with Caribou. During the year ended December 31, 2016, the Company recognized research and development expense of $1.3 million related to license and service agreements entered into with Caribou. In addition, the Company recognized general and administrative expense of $0.5 million, $0.9 million and $1.1 million during the years ended December 31, 2017, 2016 and 2015 related to the Company’s obligation to pay 30.0% of Caribou’s patent defense costs. Novartis Institutes for Biomedical Research In connection with its entry into the collaboration and license agreement and related equity transactions with Novartis, the Company issued Novartis 4,761,905 Class A-1 Preferred Units and 2,666,666 Class A-2 Preferred Units. In August 2015, Novartis acquired 761,905 shares of the Company’s Series B Preferred Stock, and in May 2016, Novartis acquired 277,777 shares of the Company’s common stock in a private placement transaction concurrent with the Company’s IPO. As a result of these and subsequent transactions, Novartis collectively owned 9.7% of the Company’s voting interests as of December 31, 2017. Refer to Note 7, Collaborations, for additional information regarding this collaboration agreement. The Company recognized collaboration revenue of $9.3 million, $7.8 million and $6.0 million in the years ended December 31, 2017, 2016 and 2015, respectively, related to this agreement. As of December 31, 2017 and 2016, the Company had recorded accounts receivable of $6.0 million and $6.0 million, respectively, and deferred revenue of $11.2 million and $11.6 million, respectively, related to this collaboration.

Summary of Significant Accounting Policies - Summary of Property and Equipment at Cost and Recognizes Depreciation and Amortization Using the Straight-Line Method Over Estimated Useful Lives (Detail)	12 Months Ended
	Dec. 31, 2017
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Office Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Computer Software [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years or term of respective lease, if shorter

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Segment
Summary Of Significant Accounting Policies [Line Items]
Percentage of likelihood of realization required to record tax benefit	50.00%
Contractual obligation	$ 0
Number of reportable segment \| Segment	1
Accounting Standards Update 2014-09 [Member] \| Difference between Revenue Guidance in Effect after Topic 606
Summary Of Significant Accounting Policies [Line Items]
Recognition of additional revenue	$ 5,400,000
Convertible Preferred Stock (Series B, Series A-2, Series A-1, Junior and Founder) [Member]
Summary Of Significant Accounting Policies [Line Items]
Loss to participating securities	$ 0

Property and Equipment, net - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment	$ 19,601	$ 12,058
Less: accumulated depreciation and amortization	(4,329)	(1,430)
Property and equipment, net	15,272	10,628
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	16,704	9,901
Office Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	960	690
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	845	703
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	656	616
Computer Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	$ 436	$ 148

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	120,000,000	120,000,000
Common stock, shares issued	42,384,623	36,018,540
Common stock, shares outstanding	42,384,623	36,018,540

The Company	12 Months Ended
The Company	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
The Company	1. The Company Intellia Therapeutics, Inc. (“Intellia” or the “Company”) is a genome editing company focused on developing curative therapeutics utilizing a biological tool known as CRISPR/Cas9. The Company commenced active operations in mid-2014. Since its inception, the Company has generated an accumulated deficit of $121.1 million through December 31, 2017 and will require substantial additional capital to fund its research and development. The Company is subject to risks and uncertainties common to early stage companies in the biotechnology industry, including, but not limited to, development by competitors of more advanced or effective therapies, dependence on key executives, protection of and dependence on proprietary technology, compliance with government regulations and ability to secure additional capital to fund operations. Programs currently under development will require significant additional research and development efforts, including preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales.

Summary of Significant Accounting Policies - Summary of Financial Assets Recognized at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 330,896	$ 270,448
Total	330,896	270,448
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	330,896	270,448
Total	$ 330,896	$ 270,448

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Employee compensation and benefits	$ 4,773	$ 2,703
Research and development and professional expenses	3,226	3,197
Accrued expenses	$ 7,999	$ 5,900

Commitments and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Jan. 31, 2016	Oct. 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Caribou [Member]
Operating Leased Assets [Line Items]
Percentage of shares owned			10.70%
Caribou [Member] \| Maximum [Member] \| Development And Regulatory Milestones [Member]
Operating Leased Assets [Line Items]
Milestone payments			$ 6,400,000
Caribou [Member] \| Maximum [Member] \| Sales Based Milestones [Member]
Operating Leased Assets [Line Items]
Milestone payments			20,000,000
Property Leases [Member]
Operating Leased Assets [Line Items]
Rent expense			6,000,000	$ 2,700,000	$ 600,000
Property Leases [Member] \| Cambridge, Massachusetts [Member]
Operating Leased Assets [Line Items]
Operating lease security deposit	$ 2,200,000	$ 300,000
Capital lease obligation additional period of term of leases in years	3 years	5 years
Operating lease arrangement term	10 years
Number of lease years before optional extension and early termination can be exercised	6 years
Prepaid lease payments			$ 3,300,000

Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Detail) - Property Leases [Member] $ in Thousands	Dec. 31, 2017 USD ($)
Operating Leased Assets [Line Items]
2018	$ 5,453
2019	5,616
2020	4,963
2021	5,507
2022	3,861
Total future minimum lease payments	$ 25,400

Equity-Based Compensation - Summary of Restricted Stock Activity, Including Converted Founder Stock (Detail)	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares, Unvested, Beginning balance \| shares	1,361,855
Number of Shares, Vested \| shares	(791,571)
Number of Shares, Forfeited \| shares	(90,462)
Number of Shares, Unvested, Ending balance \| shares	479,822
Weighted Average Grant Date Fair Value per Share, Unvested, Beginning balance \| $ / shares	$ 0.81
Weighted Average Grant Date Fair Value per Share, Vested \| $ / shares	0.69
Weighted Average Grant Date Fair Value per Share, Forfeited \| $ / shares	1.34
Weighted Average Grant Date Fair Value per Share, Unvested, Ending balance \| $ / shares	$ 0.90

Loss Per Share - Additional Information (Detail) - $ / shares			1 Months Ended
	Nov. 01, 2017	May 11, 2016	May 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Earnings Loss Per Share [Line Items]
Common stock par value per share				$ 0.0001	$ 0.0001
IPO [Member]
Earnings Loss Per Share [Line Items]
Number of common stock issued upon conversion of shares		6,900,000	6,900,000
Conversion of convertible preferred stock into common stock		23,481,956	23,481,956
Private Placement [Member]
Earnings Loss Per Share [Line Items]
Number of common stock issued upon conversion of shares		3,055,554	3,055,554
Public Offering [Member] \| Underwriting Agreement [Member]
Earnings Loss Per Share [Line Items]
Number of common stock issued upon conversion of shares	6,250,000
Common stock par value per share	$ 0.0001