Mirati Therapeutics (MRTX) 10-K 25 Feb 21 2020 FY Annual report Financial data

Cover Page

Cover Page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Feb. 19, 2021	Jun. 30, 2020
Cover [Abstract]
Entity Central Index Key	0001576263
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	--12-31
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Document Transition Report	false
Entity File Number	1-15803
Entity Registrant Name	MIRATI THERAPEUTICS, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	46-2693615
Entity Address, Address Line One	9393 Towne Centre Drive, Suite 200
Entity Address, City or Town	San Diego
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	92121
City Area Code	858
Local Phone Number	332-3410
Title of 12(b) Security	Common Stock, Par value $0.001 per share
Trading Symbol	MRTX
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 4,432.9
Entity Common Stock, Shares Outstanding		50,633,981

Recently Issued and Recently Ad

Recently Issued and Recently Adopted Accounting Pronouncements	12 Months Ended
	Dec. 31, 2020
Accounting Standards Update and Change in Accounting Principle [Abstract]
Recently Issued and Recently Adopted Accounting Pronouncements	Recently Issued and Recently Adopted Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board ("FASB") or other standard setting bodies that are adopted by the Company as of the specified effective date. Recently Adopted Accounting Pronouncements In June 2016, the FASB issued Accounting Standards Update ("ASU") 2016-13, Financial Instruments-Credit Losses: Measurement of Credit Losses on Financial Instruments , which changes the impairment model for most financial assets and certain other instruments. For trade receivables and other instruments, entities are required to use a new forward-looking expected loss model that generally results in the earlier recognition of allowances for losses. For available-for-sale debt securities with unrealized losses, the losses are recognized as allowances rather than as reductions in the amortized cost of the securities. This guidance is effective for annual reporting periods beginning after December 15, 2019, including interim periods within those years. Effective January 1, 2020, the Company adopted the provisions of ASU 2016-13. The adoption did not have a material impact on the Company's consolidated financial statements or related financial statement disclosures. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement . The new guidance modifies the disclosure requirements on fair value measurements in Topic 820. The amendments in ASU 2018-13 are effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Effective January 1, 2020, the Company adopted the provisions of ASU 2018-13. The adoption did not have a material impact on the Company's consolidated financial statements or related financial statement disclosures. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes . The update includes removing several exceptions under the existing guidance and includes several simplification updates. The guidance is effective for fiscal years beginning after December 15, 2020 and early adoption is permitted. Effective January 1, 2020, the Company early adopted this updated guidance and it did not have a material impact on the Company's consolidated financial statements or related financial statement disclosures.

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets
Cash and cash equivalents	$ 885,562	$ 46,535
Short-term investments	504,544	368,515
Other current assets	13,537	9,357
Total current assets	1,403,643	424,407
Property and equipment, net	7,809	1,776
Long-term investment	15,629	0
Right-of-use asset	39,890	582
Other long-term assets	9,157	5,435
Total assets	1,476,128	432,200
Current liabilities
Accounts payable and accrued liabilities	71,472	48,082
Deferred revenue and other current liabilities	0	824
Total current liabilities	71,472	48,906
Lease liability	41,905	0
Other liabilities	1,962	999
Total liabilities	115,339	49,905
Commitments and contingencies (see Note 15)
Shareholders' equity
Preferred stock, $0.001 par value, 10,000,000 shares authorized; none issued and outstanding at both December 31, 2020 and December 31, 2019	0	0
Common stock, $0.001 par value; 100,000,000 authorized; 50,439,069 and 39,517,329 issued and outstanding at December 31, 2020 and December 31, 2019, respectively	50	40
Additional paid-in capital	2,481,218	1,144,667
Accumulated other comprehensive income	9,759	9,889
Accumulated deficit	(1,130,238)	(772,301)
Total shareholders' equity	1,360,789	382,295
Total liabilities and shareholders' equity	$ 1,476,128	$ 432,200

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Preferred stock, par value (USD per share)	$ 0.001	$ 0.001
Preferred stock authorized (shares)	10,000,000	10,000,000
Preferred stock issued (shares)	0	0
Preferred stock outstanding (shares)	0	0
Common stock, par value (USD per share)	$ 0.001	$ 0.001
Common stock authorized (shares)	100,000,000	100,000,000
Common stock issued (shares)	50,439,069	39,517,329
Common stock outstanding (shares)	50,439,069	39,517,329

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue
Total revenue	$ 13,398	$ 3,335	$ 12,926
Expenses
Research and development	299,349	182,866	93,872
General and administrative	83,412	42,573	21,681
Total operating expenses	382,761	225,439	115,553
Loss from operations	(369,363)	(222,104)	(102,627)
Other income, net	11,426	8,848	4,209
Net loss	(357,937)	(213,256)	(98,418)
Unrealized (loss) gain on available-for-sale investments	130	(410)	0
Comprehensive loss	$ (358,067)	$ (212,846)	$ (98,418)
Basic and diluted net loss per share (USD per share)	$ (7.96)	$ (5.69)	$ (3.19)
Weighted average common shares outstanding, basic and diluted (shares)	44,987,555	37,467,505	30,897,717

CONSOLIDATED STATEMENTS OF CHAN

CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional paid-in capital	Accumulated other comprehensive income	Accumulated deficit
Balance at beginning of period (shares) at Dec. 31, 2017		28,622,886
Balance at beginning of period at Dec. 31, 2017	$ 143,288	$ 29	$ 594,407	$ 9,479	$ (460,627)
Increase (Decrease) in Stockholders' Equity
Net loss	(98,418)				(98,418)
Issuance of common stock and warrants, net of issuance costs (shares)		3,162,500
Issuance of common stock and warrants, net of issuance costs	130,663	$ 3	130,660
Share-based compensation expense	15,854		15,854
Issuance of common stock from ESPP (shares)		21,536
Issuance of common stock from ESPP	442		442
Issuance of common stock under equity incentive plans (shares)		731,935
Issuance of common stock under equity incentive plans	9,747	$ 1	9,746
Unrealized gain (loss) on investments	0
Balance at end of period (shares) at Dec. 31, 2018		32,538,857
Balance at end of period at Dec. 31, 2018	201,576	$ 33	751,109	9,479	(559,045)
Increase (Decrease) in Stockholders' Equity
Net loss	(213,256)				(213,256)
Issuance of common stock and warrants, net of issuance costs (shares)		4,269,838
Issuance of common stock and warrants, net of issuance costs	327,830	$ 4	327,826
Share-based compensation expense	55,537		55,537
Issuance of common stock from ESPP (shares)		14,488
Issuance of common stock from ESPP	675		675
Issuance of common stock under equity incentive plans (shares)		569,146
Issuance of common stock under equity incentive plans	8,473	$ 1	8,472
Proceeds from disgorgement of shareholders' short-swing profits	$ 1,050		1,050
Net exercise of warrants (shares)	2,125,000	2,125,000
Net exercise of warrants	$ 0	$ 2	(2)
Unrealized gain (loss) on investments	$ 410			410
Balance at end of period (shares) at Dec. 31, 2019	39,517,329	39,517,329
Balance at end of period at Dec. 31, 2019	$ 382,295	$ 40	1,144,667	9,889	(772,301)
Increase (Decrease) in Stockholders' Equity
Net loss	(357,937)				(357,937)
Issuance of common stock and warrants, net of issuance costs (shares)		8,124,168
Issuance of common stock and warrants, net of issuance costs	1,203,576	$ 8	1,203,568
Share-based compensation expense	85,847		85,847
Issuance of common stock from ESPP (shares)		14,436
Issuance of common stock from ESPP	$ 1,206		1,206
Issuance of common stock under equity incentive plans (shares)	1,318,963	1,319,901
Issuance of common stock under equity incentive plans	$ 45,891	$ 1	45,890
Proceeds from disgorgement of shareholders' short-swing profits	$ 41		41
Net exercise of warrants (shares)	1,400,000	1,463,235
Net exercise of warrants	$ 0	$ 1	(1)
Unrealized gain (loss) on investments	$ (130)			(130)
Balance at end of period (shares) at Dec. 31, 2020	50,439,069	50,439,069
Balance at end of period at Dec. 31, 2020	$ 1,360,789	$ 50	$ 2,481,218	$ 9,759	$ (1,130,238)

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018
Operating activities:
Net loss	$ (357,937)	$ (213,256)		$ (98,418)
Non-cash adjustments reconciling net loss to operating cash flows
Non-cash consideration earned from license agreement	(11,424)	0		0
Change in fair value on long-term investment	(4,205)	0		0
Depreciation of property and equipment	641	249		175
Accretion of discount on investments	(674)	(3,421)		(1,320)
Share-based compensation expense	85,847	55,537		15,854
Changes in operating assets and liabilities:
Other current assets	(4,180)	(5,487)		1,052
Other long-term assets	(2,842)	(4,375)		(359)
Accounts payable, accrued liabilities, deferred revenue and other liabilities	23,243	23,027		12,920
Cash flows used in operating activities	(271,531)	(147,726)		(70,096)
Investing activities:
Purchases of short-term investments	(662,824)	(530,228)		(255,795)
Sales and maturities of short-term investments	527,334	355,640		110,152
Purchases of property and equipment	(4,367)	(1,552)		(122)
Cash flows used in investing activities	(139,857)	(176,140)		(145,765)
Financing activities:
Proceeds from issuance of common stock and warrants, net of issuance costs	1,203,576	327,830	[1]	130,663
Proceeds from issuance of common stock under equity incentive plans	45,891	8,473		9,747
Proceeds from disgorgement of shareholders' short-swing profits	41	1,050		0
Proceeds from issuances under employee stock purchase plan	1,206	675		442
Cash flows provided by financing activities	1,250,714	338,028		140,852
Increase (decrease) in cash, cash equivalents and restricted cash	839,326	14,162		(75,009)
Cash, cash equivalents and restricted cash, beginning of year	46,856	32,694		107,703
Cash, cash equivalents and restricted cash, end of year	886,182	46,856		32,694
Reconciliation of cash, cash equivalents and restricted cash, end of period:
Total cash, cash equivalents and restricted cash	886,182	32,694		32,694
Supplemental disclosures of non-cash investing activities:
Accrued Capital Expenditures	292	0		0
Allowance utilized for tenant improvements	2,015	0		0
Initial recognition of operating right-of-use asset	39,890	0		0
Initial recognition of operating lease liability	$ 41,905	$ 0		$ 0
[1]	Proceeds in 2018 include pre-funded warrants to purchase up to 421,650 shares of the Company's common stock at a public offering price of $38.849 per warrant, net of issuance costs.

CONSOLIDATED STATEMENTS OF CA_2

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - $ / shares	1 Months Ended
	Oct. 31, 2020	Jan. 31, 2020	Jun. 30, 2019	Jan. 31, 2019	Jun. 30, 2018
Stock issued in transaction (shares)	4,585,706	3,538,462	2,415,000	1,854,838
Sale price of common stock (USD per share)	$ 202	$ 97.50	$ 97	$ 62
Warrant
Stock issued in transaction (shares)					421,650
Sale price of common stock (USD per share)					$ 38.849

Description of Business

Description of Business	12 Months Ended
	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of Business	Description of BusinessMirati Therapeutics, Inc. ("Mirati" or the "Company") is a clinical-stage oncology company developing product candidates to address the genetic and immunological promoters of cancer. The Company was incorporated under the laws of the State of Delaware on April 29, 2013 as Mirati Therapeutics, Inc. and is located in San Diego, California. The Company has a wholly owned subsidiary in Canada, MethylGene, Inc. ("MethylGene"), and operates primarily in the United States. The Company's common stock has been listed on the Nasdaq Global Select Market since June 5, 2018, and was previously listed on the Nasdaq Capital Market since July 15, 2013, under the ticker symbol "MRTX."

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Basis of Presentation These consolidated financial statements are prepared in accordance with accounting principles generally accepted in the United States ("U.S. GAAP"). These consolidated financial statements include the accounts of the Company and MethylGene. All significant inter-company transactions, balances and expenses have been eliminated upon consolidation. Mirati was incorporated under the laws of the State of Delaware on April 29, 2013. On May 8, 2013, the Company's Board of Directors approved and the Company entered into an arrangement agreement ("Arrangement") with MethylGene. Upon completion of the Arrangement, MethylGene became the Company's wholly-owned subsidiary. These consolidated financial statements are presented in United States ("U.S.") Dollars, which is also the functional currency of the Company. Use of Estimates The preparation of the Company's consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Reported amounts and note disclosures reflect the overall economic conditions that are most likely to occur and anticipated measures management intends to take. Actual results could differ materially from those estimates. Estimates and assumptions are reviewed quarterly. Any revisions to accounting estimates are recognized in the period in which the estimates are revised and in any future periods affected. Cash, Cash Equivalents and Short-term Investments Cash and cash equivalents consist of cash and highly liquid securities with original maturities at the date of acquisition of ninety days or less. Investments with an original maturity of more than ninety days are considered short-term investments and have been classified by management as available-for-sale. These investments are classified as current assets, even though the stated maturity date may be one year or more beyond the current consolidated balance sheet date, which reflects management’s intention to use the proceeds from sales of these securities to fund its operations, as necessary. Such investments are carried at fair value, and the unrealized gains and losses are reported as a component of accumulated other comprehensive income in shareholders' equity until realized. Realized gains and losses from the sale of available-for-sale securities, if any, are determined on a specific identification basis. Concentration of Credit Risk The Company invests its excess cash in accordance with its investment policy. The Company's investments are comprised primarily of commercial paper and debt instruments of financial institutions, corporations, U.S. government-sponsored agencies and the U.S. Treasury. The Company mitigates credit risk by maintaining a diversified portfolio and limiting the amount of investment exposure as to institution, maturity and investment type. Financial instruments that potentially subject the Company to significant credit risk consist principally of cash equivalents and short-term investments. Revenue Recognition The Company recognizes revenue in connection with certain collaboration and license agreements in accordance with the guidance of Revenue From Contracts With Customers , Accounting Standards Codification ("ASC") Topic 606 ("Topic 606"). Under Topic 606, the Company recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements the Company determines are within the scope of Topic 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. Property and Equipment, Net Property and equipment is stated at historical cost less accumulated depreciation. Historical cost includes expenditures that are directly attributable to the acquisition of the items. All repairs and maintenance are charged to consolidated net loss during the financial period in which they are incurred. Depreciation of property and equipment is calculated using the straight-line method over the estimated useful lives of the assets, as follows: Computer equipment 3 years Office and other equipment 6 years Laboratory equipment 6 years Leasehold improvements The lesser of the lease term or the life of the asset Upon disposal or impairment of property and equipment, the cost and related accumulated depreciation is removed from the consolidated financial statements and the net amount, less any proceeds, is included in consolidated net loss. Impairment of Long-Lived Assets The Company reviews its long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. If such circumstances are determined to exist, an estimate of undiscounted future cash flows produced by the long-lived asset, including its eventual residual value, is compared to the carrying value to determine whether impairment exists. In the event that such cash flows are not expected to be sufficient to recover the carrying amount of the assets, the assets are written-down to their estimated fair values. Fair value is estimated through discounted cash flow models to project cash flows from the asset. The Company recognized no impairment charges for the years ended December 31, 2020, 2019 and 2018. Share-Based Compensation Expense The Company measures and recognizes compensation expense for share-based payments based on estimated fair value, using the fair value of restricted stock units granted using the Company's closing stock price on the grant date, and using the fair value of stock options granted using the Black-Scholes option-pricing model. The Black-Scholes option-pricing model requires the use of certain estimates and judgmental assumptions that affect the amount of share-based compensation expense recognized in the Company's consolidated financial statements. These assumptions include the expected volatility of the Company's stock price, expected term of the options, the risk-free interest rate and expected dividend yields. Share-based compensation expense is recognized using the graded accelerated vesting method. Research and Development Expenses Research and development expenses are charged to consolidated net loss in the period in which they are incurred and are comprised of the following types of costs incurred in performing research and development activities: contract services for clinical trials and related clinical manufacturing costs, salaries and benefits including share-based compensation expense, costs for allocated facilities and depreciation of equipment and license fees paid in connection with the Company's early discovery efforts. General and Administrative Expenses General and administrative expenses consist primarily of salaries and related benefits, including share-based compensation expense, related to the Company's executive, finance, legal, commercial and support functions. Other general and administrative expenses include professional fees for auditing, tax, consulting and patent-related services, rent and utilities and insurance. Leases We determine if an arrangement is a lease at inception. Lease right-of-use assets represent our right to use an underlying asset for the lease term and lease liabilities represent our obligation to make lease payments arising from the lease. For operating leases with an initial term greater than 12 months, we recognize operating lease right-of-use assets and operating lease liabilities based on the present value of lease payments over the lease term at the commencement date. Operating lease right-of-use assets are comprised of the lease liability plus any lease payments made and excludes lease incentives. Lease terms include options to renew or terminate the lease when we are reasonably certain that the renewal option will be exercised or when it is reasonably certain that the termination option will not be exercised. For our operating leases, if the interest rate used to determine the present value of future lease payments it not readily determinable, we estimate our incremental borrowing rate as the discount rate for the lease. Our incremental borrowing rate is estimated to approximate the interest rate on a collateralized basis with similar terms and payments, and in similar economic environments. Lease expense for lease payments is recognized on a straight-line basis over the lease term. The Company has elected the practical expedient to not separate lease and non-lease components. Income Taxes Income taxes have been accounted for using the asset and liability method. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates applicable to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in net loss in the period that includes the enactment date. A valuation allowance against deferred tax assets is recorded if, based upon the weight of all available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. For uncertain tax positions that meet "a more likely than not" threshold, the Company recognizes the benefit of uncertain tax positions in the consolidated financial statements. Segment Reporting Operating segments are components of a business where separate discrete financial information is available for evaluation by the chief operating decision-maker for purposes of making decisions regarding resource allocation and assessing performance. To date, the Company and the chief operating decision-maker has viewed its operations and managed its business as one segment operating primarily in the United States. Net Loss per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of common shares outstanding during the period, without consideration for common share equivalents. Diluted net loss per share is computed by dividing the net loss by the weighted-average number of common shares and common share equivalents outstanding for the period. Common share equivalents outstanding, determined using the treasury stock method, are comprised of shares that may be issued under the Company’s stock option and warrant agreements. The following table presents the weighted average number of common share equivalents, calculated using the treasury stock method, not included in the calculation of diluted net loss per share due to the anti-dilutive effect of the securities: Year ended December 31, 2020 2019 2018 Common stock options 2,503,294 2,403,055 1,781,388 Common stock warrants 9,210,824 10,231,006 11,631,636 Total 11,714,118 12,634,061 13,413,024

Investments

Investments	12 Months Ended
	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Investments	Investments The following tables summarize the Company's short-term investments (in thousands): As of December 31, 2020 Maturity Amortized cost Gross unrealized gains Gross unrealized losses Estimated fair value Corporate debt securities 2 years or less $ 130,814 $ 160 $ (4) $ 130,970 Commercial paper 1 year or less 240,725 58 (18) 240,765 U.S. Agency bonds 2 years or less 83,227 37 (1) 83,263 U.S. Treasury bills 2 years or less 49,539 10 (3) 49,546 $ 504,305 $ 265 $ (26) $ 504,544 As of December 31, 2019 Maturity Amortized cost Gross unrealized gains Gross unrealized losses Estimated fair value Corporate debt securities 2 years or less $ 160,065 $ 233 $ (1) $ 160,297 Commercial paper 1 year or less 120,862 74 — 120,936 U.S. Agency bonds 2 years or less 50,745 41 (4) 50,782 U.S. Treasury bills 2 years or less 36,474 27 (1) 36,500 $ 368,146 $ 375 $ (6) $ 368,515 The Company has classified all of its short-term investments as available-for-sale as the sale of such securities may be required prior to maturity to implement management strategies, and accordingly, carries these investments at fair value. As of December 31, 2020, and December 31, 2019, aggregated gross unrealized losses of available-for-sale investments were not material, and accordingly, no allowance for credit losses was recorded.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The Company has certain financial assets and liabilities recorded at fair value which have been classified as Level 1, 2 or 3 within the fair value hierarchy as described in the accounting standards for fair value measurements. The authoritative guidance for fair value measurements defines fair value as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or the most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Market participants are buyers and sellers in the principal market that are (i) independent, (ii) knowledgeable, (iii) able to transact, and (iv) willing to transact. The guidance prioritizes the inputs used in measuring fair value into the following hierarchy: • Level 1- Quoted prices (unadjusted) in active markets for identical assets or liabilities; • Level 2- Inputs other than quoted prices included within Level 1 that are either directly or indirectly observable; and • Level 3- Unobservable inputs in which little or no market activity exists, therefore requiring an entity to develop its own assumptions about the assumptions that market participants would use in pricing. The following table summarizes the assets and liabilities measured at fair value on a recurring basis (in thousands): December 31, 2020 Total Level 1 Level 2 Level 3 Assets Cash and cash equivalents: Cash $ 20,398 $ 20,398 $ — $ — Money market funds 865,164 865,164 — — Total cash and cash equivalents 885,562 885,562 — — Short-term investments: U.S. Treasury bills 49,546 49,546 — — Corporate debt securities 130,970 — 130,970 — Commercial paper 240,765 — 240,765 — U.S. Agency bonds 83,263 — 83,263 — Total short-term investments 504,544 49,546 454,998 — Long-term investment: ORIC Pharmaceuticals 15,629 — — 15,629 Total $ 1,405,735 $ 935,108 $ 454,998 $ 15,629 December 31, 2019 Total Level 1 Level 2 Assets Cash and cash equivalents: Cash $ 662 $ 662 $ — Money market funds 45,873 45,873 — Total cash and cash equivalents 46,535 46,535 — Short-term investments: U.S. Treasury Bills 36,500 36,500 — Corporate debt securities 160,297 — 160,297 Commercial paper 120,936 — 120,936 U.S. Agency bonds 50,782 — 50,782 Total short-term investments 368,515 36,500 332,015 Total $ 415,050 $ 83,035 $ 332,015 The Company’s investments in Level 1 assets are valued based on publicly available quoted market prices for identical securities as of December 31, 2020 and 2019. The Company determines the fair value of Level 2 related securities with the aid of valuations provided by third parties using proprietary valuation models and analytical tools. These valuation models and analytical tools use market pricing or prices for similar instruments that are both objective and publicly available, including matrix pricing or reported trades, benchmark yields, broker/dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids and/or offers. The Level 3 fair value measurement of the Company's long-term investment in ORIC Pharmaceuticals, Inc., which was acquired in 2020, utilized a combination of the Asian Protective Put Option and Finnerty Put Option fair value techniques with unobservable inputs of 92% volatility and an expected term of 1.1 years to determine the discount for lack of marketability of 21.5%. See Note 9 for further details on the license agreement with ORIC Pharmaceuticals, Inc. There were no transfers between fair value measurement levels for the years ended December 31, 2020 and 2019. The following table presents the changes in estimated fair value of the Company's asset measured using significant unobservable inputs (Level 3) (in thousands): December 31, 2020 Balance - January 1, 2020 $ — Additions 11,424 Change in fair value 4,205 Balance - December 31, 2020 $ 15,629

Other Current Assets and Other

Other Current Assets and Other Long-Term Assets	12 Months Ended
	Dec. 31, 2020
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Other Current Assets and Other Long-Term Assets	Other Current Assets and Other Long-Term Assets Other current assets consisted of the following (in thousands): December 31, 2020 2019 Prepaid expenses $ 8,158 $ 5,672 Deposits and other receivables 3,075 2,119 Interest receivables 2,304 1,566 $ 13,537 $ 9,357

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): December 31, 2020 2019 Computer equipment $ 201 $ 201 Office and other equipment 329 329 Laboratory equipment 5,310 2,212 Leasehold improvements 3,639 63 Gross property and equipment 9,479 2,805 Less: Accumulated depreciation (1,670) (1,029) Property and equipment, net $ 7,809 $ 1,776

Accounts Payable and Accrued Li

Accounts Payable and Accrued Liabilities	12 Months Ended
	Dec. 31, 2020
Payables and Accruals [Abstract]
Accounts Payable and Accrued Liabilities	Accounts Payable and Accrued LiabilitiesAccounts payable and accrued liabilities consisted of the following (in thousands): December 31, 2020 2019 Accounts payable $ 18,117 $ 16,367 Accrued clinical expense 32,240 21,290 Accrued development and other expense 7,151 2,510 Accrued compensation and benefits 13,964 7,915 $ 71,472 $ 48,082

License and Collaboration Agree

License and Collaboration Agreements	12 Months Ended
	Dec. 31, 2020
Research and Development [Abstract]
License and Collaboration Agreements	License and Collaboration Agreements BeiGene Agreement Terms of Agreement On January 7, 2018, the Company and BeiGene Ltd, ("BeiGene") entered into a Collaboration and License Agreement (the “BeiGene Agreement”), pursuant to which the Company and BeiGene agreed to collaboratively develop sitravatinib in Asia (excluding Japan and certain other countries), Australia and New Zealand (the “Licensed Territory”). Under the BeiGene Agreement, the Company granted BeiGene an exclusive license to develop, manufacture and commercialize sitravatinib in the Licensed Territory, with Mirati retaining exclusive rights for the development, manufacture and commercialization of sitravatinib outside the Licensed Territory. As consideration for the rights granted to BeiGene under the BeiGene Agreement, BeiGene paid the Company a non-refundable, non-creditable up-front fee of $10.0 million. BeiGene is also required to make milestone payments to the Company of up to an aggregate of $123.0 million upon the first achievement of specified clinical, regulatory and sales milestones. The BeiGene Agreement additionally provides that BeiGene is obligated to pay to the Company royalties at tiered percentage rates ranging from mid-single digits to twenty percent on annual net sales of licensed products in the Licensed Territory, subject to reduction under specified circumstances. The BeiGene Agreement also provides that the Company will supply BeiGene with sitravatinib for use in BeiGene’s development activities in the Licensed Territory. The BeiGene Agreement will terminate upon the expiration of the last royalty term for the licensed products, which is the latest of (i) the date of expiration of the last valid patent claim related to the licensed products under the BeiGene Agreement, (ii) 10 years after the first commercial sale of a licensed product and (iii) the expiration of any regulatory exclusivity as to a licensed product. BeiGene may terminate the BeiGene Agreement at any time by providing 60 days prior written notice to the Company. Either party may terminate the BeiGene Agreement upon a material breach by the other party that remains uncured following 60 days after the date of written notice of such breach or upon certain bankruptcy events. In addition, the Company may terminate the BeiGene Agreement upon written notice to BeiGene under specified circumstances if BeiGene challenges the licensed patent rights. Revenue Recognition The Company evaluated the BeiGene Agreement under Topic 606. In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under the BeiGene Agreement, the Company performed the following steps: (i) identified the promised goods or services in the contract; (ii) determined whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measured the transaction price, including any constraints on variable consideration; (iv) allocated the transaction price to the performance obligations; and (v) recognized revenue when (or as) the Company satisfied each performance obligation. The Company determined the transaction price was equal to the up-front fee of $10.0 million. The transaction price was allocated to the performance obligations on the basis of the relative stand-alone selling price estimated for each performance obligation. In estimating the stand-alone selling price for each performance obligation, the Company developed assumptions that require judgment and included forecasted revenues, expected development timelines, discount rates, probabilities of technical and regulatory success and costs for manufacturing clinical supplies. As such, of the up-front fee, the Company allocated $9.5 million to the license of the Company's intellectual property, bundled with the associated know-how, and $0.5 million to the initial obligation to supply sitravatinib for clinical development in the Licensed Territory. Licenses of Intellectual Property. The license to the Company’s intellectual property, bundled with the associated know-how, represents a distinct performance obligation. The license and associated know-how was transferred to BeiGene during the three months ended March 31, 2018, therefore the Company recognized the full revenue related to this performance obligation in the amount of $9.5 million during the year ended December 31, 2018 as license and collaboration revenues in its consolidated statements of operations and comprehensive loss; no revenue related to this performance obligation was recorded during the years ended December 31, 2020 or 2019. Manufacturing Supply Services. The Company's initial obligation to supply sitravatinib for clinical development in the Licensed Territory represents a distinct performance obligation. The Company recognizes revenue when BeiGene obtains control of the goods, upon delivery, over the period of the obligation, which began in late 2018 and continued into 2020. The Company recognized $2.0 million as license and collaboration revenues for this performance obligation for the year ended December 31, 2020, of which $1.8 million relates to cost-sharing payments due from BeiGene, and $0.2 million relates to recognition from the deferred revenue balance. The Company recognized $3.3 million for this performance obligation during the year ended December 31, 2019, of which $3.0 million relates to cost-sharing payments due from BeiGene, and $0.3 million relates to recognition from the deferred revenue balance. The Company recognized $0.5 million for this performance obligation during the year ended December 31, 2018, of which $0.4 million relates to cost-sharing payments due from BeiGene, and $0.1 million relates to recognition from the deferred revenue balance. The Company recorded a cost-sharing receivable from BeiGene within other current assets on the consolidated balance sheets of $1.3 million and $0.4 million as of December 31, 2020 and 2019, respectively. Milestone Payments. The Company is entitled to development milestones under the agreement. The Company did not recognize revenue associated with development milestones during the years ended December 31, 2020 or 2019. During the year ended December 31, 2018, the Company recognized $3.0 million as license and collaboration revenues in connection with a milestone payment from BeiGene for initiating the first clinical trial in the Licensed Territory. The next clinical development milestone is for BeiGene initiating the first pivotal clinical trial in the Licensed Territory upon which the Company will be paid a $5.0 million milestone payment. The Company is also entitled to certain regulatory milestone payments which are paid upon receipt of regulatory approvals within the Licensed Territory. The Company determined that as of December 31, 2020, the remaining potential milestone payments are probable of significant revenue reversal as their achievement is highly dependent on factors outside the Company's control. Therefore, these payments have been fully constrained and are therefore not recognized as revenue. At the end of each subsequent reporting period, the Company will re-evaluate the probability of achievement of each milestone and any related constraint, and if necessary, adjust its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect the reported amount of license and collaboration revenues in the period of adjustment. Royalties. As the license is deemed to be the predominant item to which sales-based royalties relate, the Company will recognize revenue when the related sales occur. No royalty revenue was recognized during the years ended December 31, 2020, 2019, or 2018. The following table presents a summary of the activity in the Company's contract liabilities during the year ended December 31, 2020 (in thousands): Beginning balance, January 1, 2020 $ (172) Revenue from performance obligations satisfied during reporting period 172 Ending balance, December 31, 2020 $ — Pfizer Agreement In October 2014, the Company entered into a drug discovery collaboration and option agreement with Array BioPharma, Inc. ("Array," acquired by Pfizer Inc. ("Pfizer") during 2019) whereby Array provided services to facilitate the discovery, optimization and development of small molecule compounds that bind and specifically inhibit KRAS G12C. In June 2017, the two parties entered into a second, separate discovery collaboration and option agreement whereby Array provided services to facilitate the discovery, optimization and development of small molecule compounds that bind and specifically inhibit KRAS G12D. Both agreements established an option mechanism which enabled the Company to elect an exclusive worldwide license under the technology for the development and commercialization of certain products based on such compounds. Under the agreements, following the joint discovery periods which have concluded, the Company executed its options to retain exclusive worldwide licenses to develop, manufacture and commercialize inhibitors of KRAS G12C and KRAS G12D, including but not limited to, MRTX849 (adagrasib is the provisionally filed name for MRTX849) and MRTX1133. Under each agreement, Pfizer is entitled to potential development milestone payments of up to $9.3 million, and tiered sales milestone payments of up to $337.0 million based upon worldwide net sales, and tiered royalties in the high single digits to mid-teens on worldwide net sales of products arising from the collaborations. Under the agreements, the Company has incurred $4.5 million in development milestone payments from inception through December 31, 2020. The royalty term for each agreement shall be payable on a country-by-country and product-by-product basis, and separately will terminate at the later of (i) the date of expiration of the last valid patent claim within the collaboration patent rights or the Pfizer background technology covering such product in the country in which such product is sold at the time of such sale, or (ii) 10 years after the first commercial sale of such product in such country. The Company may terminate each agreement at any time by providing 60 days prior written notice to Pfizer. Either party may terminate each agreement upon a material breach by the other party that remains uncured following 60 days after the date of written notice of such breach or upon certain bankruptcy events. For the year ended December 31, 2020, the Company incurred expenses under these agreements with Pfizer of $4.8 million, consisting of a $3.0 million milestone payment for initiation of the first Phase 2 trial for MRTX849, a $0.3 million milestone payment for initiation of the first regulatory toxicology study for MRTX1133, and $1.5 million in research and development services. For the year ended December 31, 2019, the Company incurred expense of $7.0 million, consisting of a $1.0 million milestone payment for initiation of the first Phase 1 trial for MRTX849, and $6.0 million in research and development services. For the year ended December 31, 2018, the Company incurred $6.4 million, consisting of a $2.0 million option fee for an exclusive license to further develop and commercialize MRTX1133, a $0.3 million milestone payment for initiation of the first regulatory toxicology study for MRTX849, and $4.1 million in research and development services. ORIC Pharmaceuticals Agreement Terms of Agreement On August 3, 2020, the Company entered into a license agreement with ORIC Pharmaceuticals, Inc. ("ORIC") pursuant to which the Company granted to ORIC an exclusive, worldwide license to develop and commercialize the Company's allosteric polycomb repressive complex 2 ("PRC2") inhibitors for all indications (the "ORIC License Agreement"). In accordance with the terms of the ORIC License Agreement, in exchange for such license, ORIC issued 588,235 shares of its common stock (the "Shares") to the Company on August 3, 2020. The Shares were issued under a stock issuance agreement entered into between ORIC and the Company, dated August 3, 2020. During the eighteen Unless terminated earlier, the ORIC License Agreement will continue in effect on a country-by-country and licensed product-by-licensed product basis until the later (a) the expiration of the last valid claim of a licensed patent covering such licensed product in such country or (b) 10 years after the first commercial sale of such licensed product in such country. Following the expiration of the ORIC License Agreement, ORIC will retain its licenses under the intellectual property the Company licensed to ORIC on a royalty-free basis. The Company and ORIC may each terminate the ORIC License Agreement if the other party materially breaches the terms of such agreement, subject to specified notice and cure provisions, or enters into bankruptcy or insolvency proceedings. The Company may terminate the agreement if ORIC challenges any of the patent rights licensed to ORIC by the Company or if ORIC discontinues development of licensed products for a specified period of time. ORIC also has the right to terminate the ORIC License Agreement without cause by providing prior written notice to the Company. Revenue Recognition The Company accounted for the ORIC License Agreement under Topic 606 and identified the granting of an exclusive, worldwide license to develop and commercialize the Company's allosteric PRC2 inhibitors for all indications as a distinct performance obligation since ORIC can benefit from the license on its own by developing and commercializing the underlying product using its own resources. In determining the transaction price, the Company received the Shares as non-cash consideration. The transaction price was determined by utilizing a Level 3 fair value measurement as further described in Note 5. The Company determined the transaction price was equal to $11.4 million. The Company allocated the entire transaction price to the distinct performance obligation described above, and the license and related know-how was transferred to ORIC during the third quarter of 2020. Therefore, the Company recognized the entire transaction price of $11.4 million as license and collaboration revenues in its consolidated statements of operations and comprehensive loss.

Shareholders' Equity

Shareholders' Equity	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Shareholders' Equity	Shareholders' Equity Common Stock The following shares were reserved for future issuance: December 31, 2020 Common stock options outstanding 4,429,489 Restricted stock units outstanding 465,260 Warrants to purchase common stock 8,229,632 Employee Stock Purchase Plan 128,436 Shares available for grant 1,708,448 Total 14,961,265 Sale of Common Stock In October 2020, the Company sold 4,585,706 shares of its common stock at a public offering price of $202.00 per share. After deducting underwriter discounts, commissions and estimated offering expenses, the Company received net proceeds from the transaction of approximately $879.6 million. In January 2020, the Company sold 3,538,462 shares of its common stock at a public offering price of $97.50 per share. After deducting underwriter discounts, commissions and offering expenses, the Company received net proceeds from the transaction of $324.0 million. In June 2019, the Company sold 2,415,000 shares of its common stock at a public offering price of $97.00 per share. After deducting underwriter discounts, commissions and offering expenses, the Company received net cash proceeds from the transaction of $219.9 million. In January 2019, the Company sold 1,854,838 shares of its common stock at a public offering price of $62.00 per share. After deducting underwriter discounts, commissions and offering expenses, the Company received net cash proceeds from the transaction of $107.9 million. At-the-Market Facility On July 2, 2020, the Company entered into a sales agreement pursuant to which the Company may, from time to time, sell shares of the Company's common stock, par value $0.001 per share, having an aggregate offering price of up to $200.0 million. As of December 31, 2020, the Company has not offered or sold any shares of common stock pursuant to this sales agreement. Disgorgement Proceeds In January 2019, the Company received a payment of $1.1 million representing a disgorgement of short-swing profits from the sale of common stock by a beneficial owner pursuant to Section 16(b) of the Securities Exchange Act of 1934, as amended. In January 2020, the Company received an immaterial disgorgement of short-swing profits. The Company recognized these proceeds as a capital contribution from shareholders and reflected a corresponding increase to additional paid-in capital. Warrants As of December 31, 2020, the following warrants for common stock were issued and outstanding: Issue date Expiration date Exercise price Number of warrants outstanding January 11, 2017 None $ 0.001 3,733,218 November 20, 2017 None $ 0.001 4,137,999 June 11, 2018 None $ 0.001 358,415 8,229,632 During the year ended December 31, 2020, warrants for 1,400,012 shares of the Company's common stock were exercised via cashless exercises, resulting in the issuance of 1,400,000 shares of common stock, and warrants for 63,235 shares of the common stock were exercised for cash, generating immaterial net proceeds. During the year ended December 31, 2019, warrants for 2,125,033 shares of the Company's common stock were exercised via cashless exercise, resulting in the issuance of 2,125,000 shares of common stock. During the year ended December 31, 2018, no warrants were exercised.

Share-Based Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Share-Based Compensation	Share-Based Compensation Equity Incentive Plan The Company has a stock option plan (the "Stock Option Plan") for the benefit of employees, directors, officers and consultants of the Company. In May 2013 the Company's Board of Directors adopted the 2013 Equity Incentive Plan (the "2013 Plan"). The 2013 Plan was approved by the Company's shareholders in connection with the Arrangement. The Company's Board of Directors and shareholders approved an amendment to the 2013 Plan in 2019 to, among other things, increase the aggregate number of shares of common stock authorized for issuance under the 2013 Plan by 2.5 million shares. The 2013 Plan is a continuation of and successor to the Stock Option Plan and no further grants will be made under the Stock Option Plan. As of December 31, 2020, there were approximately 1.5 million shares available to be granted from the 2013 Plan. In December 2019, the Company's Board of Directors adopted the Inducement Plan, reserving 417,343 shares of the Company's common stock for issuance of stock options and other equity-based awards to new employees who satisfy the standards for inducement grants in accordance with the Nasdaq Stock Market LLC listing rules. As of December 31, 2020, there were 246,728 shares available to be issued from the Inducement Plan. As of December 31, 2020, share-based compensation awards under both the Stock Option Plan and the 2013 Plan consist of incentive and non-qualified stock options, and restricted stock units. Stock options granted under each of the plans must have an exercise price equal to at least 100% of the fair market value of the Company's common stock on the date of grant and generally vest over four years. Stock options granted under the Stock Option Plan have a contractual term of seven years and stock options granted under the 2013 Plan have a contractual term of ten years. Stock Options The following table summarizes the Company's stock option activity and related information for the year ended December 31, 2020: Number of Weighted Weighted-Average Remaining Contractual Term (years) Aggregate Intrinsic Value (millions) Balance outstanding as of December 31, 2019 5,287,977 $ 44.10 Granted 901,094 $ 115.83 Exercised (1,318,963) $ 34.79 Canceled/forfeited (440,619) $ 70.64 Balance outstanding as of December 31, 2020 4,429,489 $ 58.82 7.3 $ 712.5 Options exercisable at December 31, 2020 2,374,753 $ 33.62 6.2 $ 441.7 The total intrinsic value of stock options exercised was $121.5 million, $38.6 million and $18.8 million for the years ended December 31, 2020, 2019, and 2018, respectively. The Company received total cash of $45.9 million, $8.5 million and $9.7 million for the exercise of options for the years ended December 31, 2020, 2019 and 2018, respectively. The total fair value of options vested during the years ended December 31, 2020, 2019 and 2018 was $52.1 million, $20.4 million and $8.2 million, respectively. Upon option exercise, the Company issues new shares of its common stock. The fair value of options granted is estimated at the date of grant using the Black-Scholes option pricing model. Forfeitures are accounted for as incurred as a reversal of any share-based compensation expense related to options that will not vest. The assumptions used for the specified reporting periods and the resulting estimates of weighted-average estimated fair value per share of options granted during those periods are as follows: Year Ended December 31, 2020 2019 2018 Risk-free interest rate 1.1% 2.2% 2.6% Dividend yield —% —% —% Volatility factor 81.5% 82.1% 94.3% Expected term (in years) 5.6 5.6 6.0 Weighted average estimated fair value per share $77.92 $52.03 $24.39 Risk-Free Interest Rate - The risk-free interest rate is the rate for periods equal to the expected term of the stock option based on U.S. Treasury zero-coupon bonds. Dividend Yield - The dividend yield is based on the Company’s history and expectation of dividend payouts. The Company has not paid, and does not intend to pay dividends. Volatility Factor - The expected volatility assumption was determined by examining the historical volatility of the Company's stock. Expected Term - The expected term represents the weighted average period the stock options are expected to be outstanding. Prior to 2019, the Company utilized the simplified method for estimating the expected term as provided by the Securities and Exchange Commission, as the average time-to-vesting and the contractual life of the options. However, given the Company's increase in option exercise activity in recent years, the Company believes it has sufficient history to determine an expected term based on past exercise activity. Starting in 2019, the Company utilized historical exercise activity to estimate the expected term. The change in calculation of the expected term did not have a material impact on the Company's consolidated financial statements or related financial statement disclosures. The total compensation cost not yet recognized as of December 31, 2020 related to non-vested option awards was $61.4 million which will be recognized over a weighted-average period of 1.2 years. Restricted Stock Units ("RSUs") The Company began issuing RSUs during the year ended December 31, 2020. The RSUs generally vest annually over four years and are subject to continued service. A summary of the Company's RSU activity for the year ended December 31, 2020 is as follows: Number of Weighted average grant date fair value Aggregate Intrinsic Value (millions) Balance outstanding as of December 31, 2019 — $ — Granted 487,569 $ 113.91 Releases (938) $ 113.20 Canceled/forfeited (21,371) $ 109.00 Balance outstanding as of December 31, 2020 465,260 $ 114.14 $ 102.2 The total vest date fair value of RSUs that vested during the year ended December 31, 2020 was $0.1 million. The total compensation cost not yet recognized as of December 31, 2020 related to non-vested RSUs was $31.4 million, which will be recognized over a weighted-average period of 1.7 years. Total share-based compensation expense by consolidated statement of operations and comprehensive loss classification is presented below (in thousands): Year ended December 31, 2020 2019 2018 Research and development expense $ 48,044 $ 31,024 $ 7,232 General and administrative expense 37,803 24,513 8,622 $ 85,847 $ 55,537 $ 15,854 For the years ended December 31, 2020, 2019 and 2018, no share-based compensation expense was capitalized and there were no recognized tax benefits associated with the share-based compensation charge. 2013 Employee Stock Purchase Plan In May 2013, the Company's Board of Directors adopted the 2013 Employee Stock Purchase Plan (the "ESPP"). The ESPP was approved by the Company's shareholders in connection with the Arrangement. In December 2014, the ESPP became effective and the first purchase period began. The ESPP permits eligible employees to make payroll deductions to purchase up to $25,000 of the Company’s common stock on regularly scheduled purchase dates at a discount. Offering periods under the ESPP are not more than six months in duration and shares are purchased at 85% of the lower of the closing price for the Company’s common stock on the first day of the offering period or the date of purchase. The ESPP initially authorized the issuance of 300,000 shares of the Company’s common stock pursuant to rights granted to employees for their payroll deductions. As of December 31, 2020, 171,564 shares have been issued out of the plan.

Employee Benefit Plan

Employee Benefit Plan	12 Months Ended
	Dec. 31, 2020
Retirement Benefits [Abstract]
Employee Benefit Plan	Employee Benefit Plan    The Company has a defined contribution 401(k) plan (the "Plan") for all employees. Employees are eligible to participate in the Plan if they are at least 21 years of age or older. Under the terms of the Plan, employees may make voluntary contributions as a percentage of compensation. During the year ended December 31, 2020, the Company matched up to 5% of an employee's earnings, subject to Internal Revenue Service limitations. In 2019, the Company matched up to 4% of an employee's contributions, subject to a limit of $2,500. Expense associated with the Company's matching contribution totaled $1.3 million, $0.2 million, and $0.1 million for the years ended December 31, 2020, 2019, and 2018 respectively.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company had no federal income tax expense and immaterial state tax expense for the years ended December 31, 2020, 2019 and 2018. The differences between the effective income tax rate and the statutory tax rates during the years ended 2020, 2019 and 2018 are as follows (in thousands): Year Ended December 31, 2020 2019 2018 Net loss before tax $ (357,937) $ (213,256) $ (98,418) Statutory combined U.S. federal and state tax rate 21.00 % 21.00 % 21.00 % Statutory federal and state taxes (75,167) (44,784) (20,668) State income taxes, net of federal benefit (13,490) — — Increase (decrease) in taxes recoverable resulting from: Effect of change in valuation allowance 110,985 52,719 25,959 Non-deductible share-based compensation 2,724 1,810 884 Tax deductions for share-based compensation (17,991) (6,917) (2,924) Tax credits (15,672) (8,621) (5,130) Uncertain tax positions 3,857 2,143 1,283 Return to provision and other true-ups 25 (60) 375 Non-deductible officers' compensation 4,697 3,527 179 Other differences 32 183 42 Income tax benefit $ — $ — $ — Deferred Tax The following table summarizes the significant components of the Company's deferred tax assets (in thousands): December 31, 2020 2019 Deferred tax assets: Tangible and intangible depreciable assets $ 32,180 $ 6,978 Stock compensation 19,183 12,321 Provisions 2,510 934 Other, net — 35 Lease liability 8,800 137 Net operating loss carry forwards 182,536 116,345 Capital loss carryforward 114 178 Canada scientific research and experimental development expenditures 5,471 5,471 U.S. research and development tax credits 28,834 17,080 Total gross deferred tax assets 279,628 159,479 Less valuation allowance (270,368) (159,357) Net deferred tax assets $ 9,260 $ 122 Deferred tax liabilities: Right-of-use asset $ (8,377) $ (122) Non-current investment (883) — Net deferred income taxes $ — $ — Total valuation allowance increased by $111.0 million for the year ended December 31, 2020. The Company has established a full valuation allowance against its net deferred tax assets as of December 31, 2020 due to the uncertainty surrounding the realization of such assets as evidenced by the cumulative losses from operations through December 31, 2020. For Canadian federal income tax purposes, the Company's Canadian federal scientific research and experimental development expenditures amounted to $19.9 million at December 31, 2020, 2019 and 2018 and for provincial income tax purposes amounted to $21.6 million at December 31, 2020, 2019 and 2018. As operations in Canada ceased during 2014, no expenditures were incurred for the years ended December 31, 2020, 2019 and 2018. These expenditures are available to reduce future taxable income and have an unlimited carry forward period. Scientific research and development expenditures are subject to verification by the taxation authorities, and accordingly, these amounts may vary by a material amount. In addition, the Company has research and development tax credit carryforwards for U.S. federal and state income tax purposes as of December 31, 2020 of $29.7 million and $11.3 million, respectively. The federal credits will begin to expire in 2033 unless utilized and the state credits have an indefinite life. At December 31, 2020, the Company's net operating loss carry forwards ("NOLs") for U.S. federal and state income taxes were $702.9 million and $274.3 million, respectively and the Company's NOLs for Canadian federal and provincial income tax purposes were $79.3 million and $78.7 million, respectively. The NOLs expire as follows (in thousands): US Canada Federal State Federal Provincial Expires in: 2030 $ — $ — $ 4,830 $ 4,907 2031 — — 7,059 7,066 2032 — — 13,308 12,433 2033 2,225 2,232 18,623 19,385 2034 7,276 22,162 32,401 31,809 2035 53,359 52,950 1,084 1,084 2036 23,379 — 777 777 2037 65,509 — 697 697 2038 — 3,817 — — 2039 — — 242 242 2040 — 193,163 273 273 Does not expire 551,134 — — — $ 702,882 $ 274,324 $ 79,294 $ 78,673 The future utilization of the U.S. federal and state NOL carryforwards to offset future taxable income may be subject to an annual limitation as a result of ownership changes that may have occurred previously or may occur in the future. The Tax Reform Act of 1986 (the "Act") limits a company's ability to utilize certain tax credit carryforwards and net operating loss carryforwards in the event of a cumulative change in ownership in excess of 50% (by value) as defined in the Act. During 2017, the Company completed a study to assess whether an ownership change, as defined by Section 382 of the Act, had occurred from the Company’s formation through December 31, 2017. The results of the study have been extended through December 31, 2020. Based upon the study, the Company determined an ownership change had occurred during 2017, causing the annual utilization of the NOL and credit carryforwards to be limited. The Company does not believe any of the NOL and credit carryforwards generated through December 31, 2020 would expire solely as a result of annual limitations on the utilization of those attributes. The Canadian Federal and Provincial Tax Acts maintain similar rules in the case of acquisition of control, which may limit the utilization of tax attributes. The Company files income tax returns in the U.S. (federal and state) and Canada (federal and provincial). The Company’s U.S. operations have not been audited for any open taxation years. The Company has experienced losses for U.S. tax purposes and therefore, the taxation authorities may review any loss year, if and when the losses are utilized. For Canadian tax purposes, the Company remains subject to federal and provincial audit for the December 31, 2016 and subsequent taxable years. Where tax years remain open, the Company considers it reasonably possible that issues may be raised or tax positions agreed to with the taxation authorities, which may result in increases or decreases of the balance of non-refundable investment tax credits ("ITCs") and NOLs. However, an estimate of such increases and decreases cannot be currently made. A reconciliation of the beginning and ending amounts of unrecognized tax positions are as follows (in thousands): Federal Provincial/State December 31, December 31, 2020 2019 2018 2020 2019 2018 Unrecognized tax positions, beginning of year $ 4,268 $ 2,617 $ 1,693 $ 8,648 $ 8,010 $ 7,556 Gross increase — current period tax positions 3,126 1,651 924 1,004 638 454 Gross decrease — prior period tax positions — — — — — — Gross increase — prior period tax positions — — — — — — Expiration of statute of limitations — — — — — — Unrecognized tax positions, end of year $ 7,394 $ 4,268 $ 2,617 $ 9,652 $ 8,648 $ 8,010 If recognized, none of the unrecognized tax positions would impact the Company's income tax benefit or effective tax rate as long as the Company's net deferred tax assets remain subject to a full valuation allowance. The Company does not

Investment Tax Credits

Investment Tax Credits	12 Months Ended
	Dec. 31, 2020
INVESTMENT TAX CREDITS
Investment Tax Credits	Investment Tax Credits In prior years, the Company was entitled to claim Canadian federal and provincial ITCs for eligible scientific research and development expenditures. The Company recorded ITCs based on management's best estimates of the amount to be recovered and ITCs claimed are subject to audit by the taxation authorities and accordingly, may vary by a material amount. The Company has not recorded federal or provincial ITCs since the year ended December 31, 2013, as the primary operations of the Company were moved from Canada to San Diego, California in early 2014. The Company's non-refundable Canadian federal ITCs as of December 31, 2020 are $3.9 million and relate to scientific research and development expenditures, which may be utilized to reduce Canadian federal income taxes payable in future years. The benefits of the non-refundable Canadian federal ITCs have not been recognized in the financial statements and will be recorded as a reduction of tax expense when realized. The non-refundable investment tax credits expire as follows (in thousands): Federal ITC Expires in: 2030 $ 764 2031 1,000 2032 1,125 2033 1,031 $ 3,920

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies On June 30, 2020, the Company entered into an amended and restated lease agreement (the "Amended and Restated Lease") for office and laboratory space located in San Diego, California, for the Company's new corporate headquarters. The Amended and Restated Lease supersedes in its entirety the original lease agreement for the Company's future corporate headquarters dated as of August 22, 2019. The Amended and Restated Lease has a lease term of approximately 12 years ("Lease Term"), unless terminated earlier. The Lease Term has an initial abatement period, and the initial base rent payable will be approximately $0.6 million per month following the abatement period, which amount will increase by 3% per year over the Lease Term. The Company has also received incentives from the landlord for tenant improvements. As of December 31, 2020, the underlying asset was available for use by the Company to construct tenant improvements and therefore, the Lease Term is considered to have commenced. The Amended and Restated Lease is considered to be an operating lease, and the Amended and Restated Lease indicates the interest rate applicable to the lease is 12%, therefore the Company used a discount rate of 12% to calculate the present value of its lease payments over the Lease Term. As of December 31, 2020, the consolidated balance sheet includes an operating right-of-use asset of $39.9 million and an operating lease liability of $41.9 million. For the year ended December 31, 2020, the Company recorded $0.3 million in operating lease expense. As of December 31, 2020, the approximate future minimum lease payments under the Amended and Restated Lease are as follows: (in thousands) Operating Lease Year Ending December 31, 2021 $ — 2022 1,681 2023 7,844 2024 8,080 2025 8,322 Thereafter 68,257 Total operating lease payments 94,184 Less: Amount representing interest (52,279) Present value of operating lease payments $ 41,905 On June 24, 2014, the Company entered into a lease agreement for completed office and laboratory space located in San Diego, California. The office space under the lease is the Company's corporate headquarters. The lease commenced in two phases (in July 2014 and March 2015) at a combined total initial monthly rent of $24,100 per month. The leased property is subject to a 3% annual rent increase following availability. In addition to such base monthly rent, the Company is obligated to pay certain customary amounts for its share of operating expenses and facility amenities. The original lease provided for expiration on January 31, 2018, and the Company entered into subsequent amendments to the original lease to extend the lease term to mid-2021 and expand the size of the existing space. All other terms and covenants from the original lease agreement remain unchanged.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data (Unaudited)	Selected Quarterly Financial Data (Unaudited) The following is a summary of the quarterly results of the Company for the years ended December 31, 2020 and 2019 (unaudited, in thousands, except per share data): Three Months Ended 3/31/2020 6/30/2020 9/30/2020 12/31/2020 License and collaboration revenues $ 267 $ — $ 11,424 $ 1,707 Loss from operations (89,487) (84,862) (88,678) (106,336) Net loss (86,655) (82,859) (87,336) (101,087) Basic and diluted net loss per share $ (2.02) $ (1.89) $ (1.96) $ (2.08) Three Months Ended 3/31/2019 6/30/2019 9/30/2019 12/31/2019 License and collaboration revenues $ 1,244 $ 577 $ 988 $ 526 Loss from operations (42,758) (47,641) (57,059) (74,646) Net loss (40,912) (45,695) (54,273) (72,376) Basic and diluted net loss per share $ (1.17) $ (1.26) $ (1.38) $ (1.83) Net loss per share is computed independently for each of the quarters presented. Therefore, the sum of the quarterly per-share calculations will not necessarily equal the annual per share calculation.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation These consolidated financial statements are prepared in accordance with accounting principles generally accepted in the United States ("U.S. GAAP"). These consolidated financial statements include the accounts of the Company and MethylGene. All significant inter-company transactions, balances and expenses have been eliminated upon consolidation. Mirati was incorporated under the laws of the State of Delaware on April 29, 2013. On May 8, 2013, the Company's Board of Directors approved and the Company entered into an arrangement agreement ("Arrangement") with MethylGene. Upon completion of the Arrangement, MethylGene became the Company's wholly-owned subsidiary. These consolidated financial statements are presented in United States ("U.S.") Dollars, which is also the functional currency of the Company.
Use of Estimates	Use of Estimates The preparation of the Company's consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Reported amounts and note disclosures reflect the overall economic conditions that are most likely to occur and anticipated measures management intends to take. Actual results could differ materially from those estimates. Estimates and assumptions are reviewed quarterly. Any revisions to accounting estimates are recognized in the period in which the estimates are revised and in any future periods affected.
Cash, Cash Equivalents, and Short-term Investments	Cash, Cash Equivalents and Short-term InvestmentsCash and cash equivalents consist of cash and highly liquid securities with original maturities at the date of acquisition of ninety days or less. Investments with an original maturity of more than ninety days are considered short-term investments and have been classified by management as available-for-sale. These investments are classified as current assets, even though the stated maturity date may be one year or more beyond the current consolidated balance sheet date, which reflects management’s intention to use the proceeds from sales of these securities to fund its operations, as necessary. Such investments are carried at fair value, and the unrealized gains and losses are reported as a component of accumulated other comprehensive income in shareholders' equity until realized. Realized gains and losses from the sale of available-for-sale securities, if any, are determined on a specific identification basis.
Concentration of Credit Risk	Concentration of Credit Risk The Company invests its excess cash in accordance with its investment policy. The Company's investments are comprised primarily of commercial paper and debt instruments of financial institutions, corporations, U.S. government-sponsored agencies and the U.S. Treasury. The Company mitigates credit risk by maintaining a diversified portfolio and limiting the amount of investment exposure as to institution, maturity and investment type. Financial instruments that potentially subject the Company to significant credit risk consist principally of cash equivalents and short-term investments.
Revenue Recognition	Revenue Recognition The Company recognizes revenue in connection with certain collaboration and license agreements in accordance with the guidance of Revenue From Contracts With Customers , Accounting Standards Codification ("ASC") Topic 606 ("Topic 606"). Under Topic 606, the Company recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements the Company determines are within the scope of Topic 606, the Company performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the Company satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied.
Property and Equipment, Net	Property and Equipment, Net Property and equipment is stated at historical cost less accumulated depreciation. Historical cost includes expenditures that are directly attributable to the acquisition of the items. All repairs and maintenance are charged to consolidated net loss during the financial period in which they are incurred. Depreciation of property and equipment is calculated using the straight-line method over the estimated useful lives of the assets, as follows: Computer equipment 3 years Office and other equipment 6 years Laboratory equipment 6 years Leasehold improvements The lesser of the lease term or the life of the asset Upon disposal or impairment of property and equipment, the cost and related accumulated depreciation is removed from the consolidated financial statements and the net amount, less any proceeds, is included in consolidated net loss.
Impairment of Long-Lived Assets	Impairment of Long-Lived AssetsThe Company reviews its long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. If such circumstances are determined to exist, an estimate of undiscounted future cash flows produced by the long-lived asset, including its eventual residual value, is compared to the carrying value to determine whether impairment exists. In the event that such cash flows are not expected to be sufficient to recover the carrying amount of the assets, the assets are written-down to their estimated fair values. Fair value is estimated through discounted cash flow models to project cash flows from the asset.
Share-Based Compensation Expense	Share-Based Compensation ExpenseThe Company measures and recognizes compensation expense for share-based payments based on estimated fair value, using the fair value of restricted stock units granted using the Company's closing stock price on the grant date, and using the fair value of stock options granted using the Black-Scholes option-pricing model. The Black-Scholes option-pricing model requires the use of certain estimates and judgmental assumptions that affect the amount of share-based compensation expense recognized in the Company's consolidated financial statements. These assumptions include the expected volatility of the Company's stock price, expected term of the options, the risk-free interest rate and expected dividend yields. Share-based compensation expense is recognized using the graded accelerated vesting method.
Research and Development Expenses	Research and Development Expenses Research and development expenses are charged to consolidated net loss in the period in which they are incurred and are comprised of the following types of costs incurred in performing research and development activities: contract services for
General and Administrative Expenses	General and Administrative ExpensesGeneral and administrative expenses consist primarily of salaries and related benefits, including share-based compensation expense, related to the Company's executive, finance, legal, commercial and support functions. Other general and administrative expenses include professional fees for auditing, tax, consulting and patent-related services, rent and utilities and insurance.
Leases	Leases We determine if an arrangement is a lease at inception. Lease right-of-use assets represent our right to use an underlying asset for the lease term and lease liabilities represent our obligation to make lease payments arising from the lease. For operating leases with an initial term greater than 12 months, we recognize operating lease right-of-use assets and operating lease liabilities based on the present value of lease payments over the lease term at the commencement date. Operating lease right-of-use assets are comprised of the lease liability plus any lease payments made and excludes lease incentives. Lease terms include options to renew or terminate the lease when we are reasonably certain that the renewal option will be exercised or when it is reasonably certain that the termination option will not be exercised. For our operating leases, if the interest rate used to determine the present value of future lease payments it not readily determinable, we estimate our incremental borrowing rate as the discount rate for the lease. Our incremental borrowing rate is estimated to approximate the interest rate on a collateralized basis with similar terms and payments, and in similar economic environments. Lease expense for lease payments is recognized on a straight-line basis over the lease term. The Company has elected the practical expedient to not separate lease and non-lease components.
Income Taxes	Income Taxes Income taxes have been accounted for using the asset and liability method. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates applicable to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in net loss in the period that includes the enactment date. A valuation allowance against deferred tax assets is recorded if, based upon the weight of all available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. For uncertain tax positions that meet "a more likely than not" threshold, the Company recognizes the benefit of uncertain tax positions in the consolidated financial statements.
Segment Reporting	Segment Reporting Operating segments are components of a business where separate discrete financial information is available for evaluation by the chief operating decision-maker for purposes of making decisions regarding resource allocation and assessing performance. To date, the Company and the chief operating decision-maker has viewed its operations and managed its business as one segment operating primarily in the United States.
Net Loss per Share	Net Loss per Share Basic net loss per share is calculated by dividing the net loss by the weighted-average number of common shares outstanding during the period, without consideration for common share equivalents. Diluted net loss per share is computed by dividing the net loss by the weighted-average number of common shares and common share equivalents outstanding for the period. Common share equivalents outstanding, determined using the treasury stock method, are comprised of shares that may be issued under the Company’s stock option and warrant agreements.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In June 2016, the FASB issued Accounting Standards Update ("ASU") 2016-13, Financial Instruments-Credit Losses: Measurement of Credit Losses on Financial Instruments , which changes the impairment model for most financial assets and certain other instruments. For trade receivables and other instruments, entities are required to use a new forward-looking expected loss model that generally results in the earlier recognition of allowances for losses. For available-for-sale debt securities with unrealized losses, the losses are recognized as allowances rather than as reductions in the amortized cost of the securities. This guidance is effective for annual reporting periods beginning after December 15, 2019, including interim periods within those years. Effective January 1, 2020, the Company adopted the provisions of ASU 2016-13. The adoption did not have a material impact on the Company's consolidated financial statements or related financial statement disclosures. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement . The new guidance modifies the disclosure requirements on fair value measurements in Topic 820. The amendments in ASU 2018-13 are effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Effective January 1, 2020, the Company adopted the provisions of ASU 2018-13. The adoption did not have a material impact on the Company's consolidated financial statements or related financial statement disclosures. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes . The update includes removing several exceptions under the existing guidance and includes several simplification updates. The guidance is effective for fiscal years beginning after December 15, 2020 and early adoption is permitted. Effective January 1, 2020, the Company early adopted this updated guidance and it did not have a material impact on the Company's consolidated financial statements or related financial statement disclosures.
Fair Value Measurements	The authoritative guidance for fair value measurements defines fair value as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or the most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Market participants are buyers and sellers in the principal market that are (i) independent, (ii) knowledgeable, (iii) able to transact, and (iv) willing to transact. The guidance prioritizes the inputs used in measuring fair value into the following hierarchy: • Level 1- Quoted prices (unadjusted) in active markets for identical assets or liabilities; • Level 2- Inputs other than quoted prices included within Level 1 that are either directly or indirectly observable; and • Level 3- Unobservable inputs in which little or no market activity exists, therefore requiring an entity to develop its own assumptions about the assumptions that market participants would use in pricing.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Schedule of Estimated Useful Lives of Assets	Depreciation of property and equipment is calculated using the straight-line method over the estimated useful lives of the assets, as follows: Computer equipment 3 years Office and other equipment 6 years Laboratory equipment 6 years Leasehold improvements The lesser of the lease term or the life of the asset
Schedule of Potentially Dilutive Securities Excluded from Earnings Per Share Calculation	The following table presents the weighted average number of common share equivalents, calculated using the treasury stock method, not included in the calculation of diluted net loss per share due to the anti-dilutive effect of the securities: Year ended December 31, 2020 2019 2018 Common stock options 2,503,294 2,403,055 1,781,388 Common stock warrants 9,210,824 10,231,006 11,631,636 Total 11,714,118 12,634,061 13,413,024

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Schedule of Short-Term Investments	The following tables summarize the Company's short-term investments (in thousands): As of December 31, 2020 Maturity Amortized cost Gross unrealized gains Gross unrealized losses Estimated fair value Corporate debt securities 2 years or less $ 130,814 $ 160 $ (4) $ 130,970 Commercial paper 1 year or less 240,725 58 (18) 240,765 U.S. Agency bonds 2 years or less 83,227 37 (1) 83,263 U.S. Treasury bills 2 years or less 49,539 10 (3) 49,546 $ 504,305 $ 265 $ (26) $ 504,544 As of December 31, 2019 Maturity Amortized cost Gross unrealized gains Gross unrealized losses Estimated fair value Corporate debt securities 2 years or less $ 160,065 $ 233 $ (1) $ 160,297 Commercial paper 1 year or less 120,862 74 — 120,936 U.S. Agency bonds 2 years or less 50,745 41 (4) 50,782 U.S. Treasury bills 2 years or less 36,474 27 (1) 36,500 $ 368,146 $ 375 $ (6) $ 368,515

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis	The following table summarizes the assets and liabilities measured at fair value on a recurring basis (in thousands): December 31, 2020 Total Level 1 Level 2 Level 3 Assets Cash and cash equivalents: Cash $ 20,398 $ 20,398 $ — $ — Money market funds 865,164 865,164 — — Total cash and cash equivalents 885,562 885,562 — — Short-term investments: U.S. Treasury bills 49,546 49,546 — — Corporate debt securities 130,970 — 130,970 — Commercial paper 240,765 — 240,765 — U.S. Agency bonds 83,263 — 83,263 — Total short-term investments 504,544 49,546 454,998 — Long-term investment: ORIC Pharmaceuticals 15,629 — — 15,629 Total $ 1,405,735 $ 935,108 $ 454,998 $ 15,629 December 31, 2019 Total Level 1 Level 2 Assets Cash and cash equivalents: Cash $ 662 $ 662 $ — Money market funds 45,873 45,873 — Total cash and cash equivalents 46,535 46,535 — Short-term investments: U.S. Treasury Bills 36,500 36,500 — Corporate debt securities 160,297 — 160,297 Commercial paper 120,936 — 120,936 U.S. Agency bonds 50,782 — 50,782 Total short-term investments 368,515 36,500 332,015 Total $ 415,050 $ 83,035 $ 332,015
Schedule of Changes in Estimated Fair Value of Assets with Unobservable Inputs	The following table presents the changes in estimated fair value of the Company's asset measured using significant unobservable inputs (Level 3) (in thousands): December 31, 2020 Balance - January 1, 2020 $ — Additions 11,424 Change in fair value 4,205 Balance - December 31, 2020 $ 15,629

Other Current Assets and Othe_2

Other Current Assets and Other Long-Term Assets (Tables)	12 Months Ended
	Dec. 31, 2020
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Schedule of Other Current Assets	Other current assets consisted of the following (in thousands): December 31, 2020 2019 Prepaid expenses $ 8,158 $ 5,672 Deposits and other receivables 3,075 2,119 Interest receivables 2,304 1,566 $ 13,537 $ 9,357

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment, net consisted of the following (in thousands): December 31, 2020 2019 Computer equipment $ 201 $ 201 Office and other equipment 329 329 Laboratory equipment 5,310 2,212 Leasehold improvements 3,639 63 Gross property and equipment 9,479 2,805 Less: Accumulated depreciation (1,670) (1,029) Property and equipment, net $ 7,809 $ 1,776

Accounts Payable and Accrued _2

Accounts Payable and Accrued Liabilities (Tables)	12 Months Ended
	Dec. 31, 2020
Payables and Accruals [Abstract]
Schedule of Accounts Payable and Accrued Liabilities	Accounts payable and accrued liabilities consisted of the following (in thousands): December 31, 2020 2019 Accounts payable $ 18,117 $ 16,367 Accrued clinical expense 32,240 21,290 Accrued development and other expense 7,151 2,510 Accrued compensation and benefits 13,964 7,915 $ 71,472 $ 48,082

License and Collaboration Agr_2

License and Collaboration Agreements (Tables)	12 Months Ended
	Dec. 31, 2020
Research and Development [Abstract]
Schedule of Activity in Contract Liabilities	The following table presents a summary of the activity in the Company's contract liabilities during the year ended December 31, 2020 (in thousands): Beginning balance, January 1, 2020 $ (172) Revenue from performance obligations satisfied during reporting period 172 Ending balance, December 31, 2020 $ —

Shareholders' Equity (Tables)

Shareholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Schedule of Shares Reserved for Future Issuance	The following shares were reserved for future issuance: December 31, 2020 Common stock options outstanding 4,429,489 Restricted stock units outstanding 465,260 Warrants to purchase common stock 8,229,632 Employee Stock Purchase Plan 128,436 Shares available for grant 1,708,448 Total 14,961,265
Schedule of Warrants Issued and Outstanding	As of December 31, 2020, the following warrants for common stock were issued and outstanding: Issue date Expiration date Exercise price Number of warrants outstanding January 11, 2017 None $ 0.001 3,733,218 November 20, 2017 None $ 0.001 4,137,999 June 11, 2018 None $ 0.001 358,415 8,229,632

Share-Based Compensation (Table

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Schedule of Stock Option Activity	The following table summarizes the Company's stock option activity and related information for the year ended December 31, 2020: Number of Weighted Weighted-Average Remaining Contractual Term (years) Aggregate Intrinsic Value (millions) Balance outstanding as of December 31, 2019 5,287,977 $ 44.10 Granted 901,094 $ 115.83 Exercised (1,318,963) $ 34.79 Canceled/forfeited (440,619) $ 70.64 Balance outstanding as of December 31, 2020 4,429,489 $ 58.82 7.3 $ 712.5 Options exercisable at December 31, 2020 2,374,753 $ 33.62 6.2 $ 441.7
Schedule of Assumptions Used in Fair Value Estimates	The assumptions used for the specified reporting periods and the resulting estimates of weighted-average estimated fair value per share of options granted during those periods are as follows: Year Ended December 31, 2020 2019 2018 Risk-free interest rate 1.1% 2.2% 2.6% Dividend yield —% —% —% Volatility factor 81.5% 82.1% 94.3% Expected term (in years) 5.6 5.6 6.0 Weighted average estimated fair value per share $77.92 $52.03 $24.39
Schedule of Share-based Compensation Expense	Total share-based compensation expense by consolidated statement of operations and comprehensive loss classification is presented below (in thousands): Year ended December 31, 2020 2019 2018 Research and development expense $ 48,044 $ 31,024 $ 7,232 General and administrative expense 37,803 24,513 8,622 $ 85,847 $ 55,537 $ 15,854
Schedule of Restricted Stock Units Activity	A summary of the Company's RSU activity for the year ended December 31, 2020 is as follows: Number of Weighted average grant date fair value Aggregate Intrinsic Value (millions) Balance outstanding as of December 31, 2019 — $ — Granted 487,569 $ 113.91 Releases (938) $ 113.20 Canceled/forfeited (21,371) $ 109.00 Balance outstanding as of December 31, 2020 465,260 $ 114.14 $ 102.2

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Reconciliation	The differences between the effective income tax rate and the statutory tax rates during the years ended 2020, 2019 and 2018 are as follows (in thousands): Year Ended December 31, 2020 2019 2018 Net loss before tax $ (357,937) $ (213,256) $ (98,418) Statutory combined U.S. federal and state tax rate 21.00 % 21.00 % 21.00 % Statutory federal and state taxes (75,167) (44,784) (20,668) State income taxes, net of federal benefit (13,490) — — Increase (decrease) in taxes recoverable resulting from: Effect of change in valuation allowance 110,985 52,719 25,959 Non-deductible share-based compensation 2,724 1,810 884 Tax deductions for share-based compensation (17,991) (6,917) (2,924) Tax credits (15,672) (8,621) (5,130) Uncertain tax positions 3,857 2,143 1,283 Return to provision and other true-ups 25 (60) 375 Non-deductible officers' compensation 4,697 3,527 179 Other differences 32 183 42 Income tax benefit $ — $ — $ —
Schedule of Deferred Tax	The following table summarizes the significant components of the Company's deferred tax assets (in thousands): December 31, 2020 2019 Deferred tax assets: Tangible and intangible depreciable assets $ 32,180 $ 6,978 Stock compensation 19,183 12,321 Provisions 2,510 934 Other, net — 35 Lease liability 8,800 137 Net operating loss carry forwards 182,536 116,345 Capital loss carryforward 114 178 Canada scientific research and experimental development expenditures 5,471 5,471 U.S. research and development tax credits 28,834 17,080 Total gross deferred tax assets 279,628 159,479 Less valuation allowance (270,368) (159,357) Net deferred tax assets $ 9,260 $ 122 Deferred tax liabilities: Right-of-use asset $ (8,377) $ (122) Non-current investment (883) — Net deferred income taxes $ — $ —
Schedule of Expiration of NOLs	The NOLs expire as follows (in thousands): US Canada Federal State Federal Provincial Expires in: 2030 $ — $ — $ 4,830 $ 4,907 2031 — — 7,059 7,066 2032 — — 13,308 12,433 2033 2,225 2,232 18,623 19,385 2034 7,276 22,162 32,401 31,809 2035 53,359 52,950 1,084 1,084 2036 23,379 — 777 777 2037 65,509 — 697 697 2038 — 3,817 — — 2039 — — 242 242 2040 — 193,163 273 273 Does not expire 551,134 — — — $ 702,882 $ 274,324 $ 79,294 $ 78,673
Schedule of Unrecognized Tax Positions	A reconciliation of the beginning and ending amounts of unrecognized tax positions are as follows (in thousands): Federal Provincial/State December 31, December 31, 2020 2019 2018 2020 2019 2018 Unrecognized tax positions, beginning of year $ 4,268 $ 2,617 $ 1,693 $ 8,648 $ 8,010 $ 7,556 Gross increase — current period tax positions 3,126 1,651 924 1,004 638 454 Gross decrease — prior period tax positions — — — — — — Gross increase — prior period tax positions — — — — — — Expiration of statute of limitations — — — — — — Unrecognized tax positions, end of year $ 7,394 $ 4,268 $ 2,617 $ 9,652 $ 8,648 $ 8,010

Investment Tax Credits (Tables)

Investment Tax Credits (Tables)	12 Months Ended
	Dec. 31, 2020
INVESTMENT TAX CREDITS
Schedule of Non-Refundable Investment Tax Credits	The non-refundable investment tax credits expire as follows (in thousands): Federal ITC Expires in: 2030 $ 764 2031 1,000 2032 1,125 2033 1,031 $ 3,920

Commitment and Contingencies (T

Commitment and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments	As of December 31, 2020, the approximate future minimum lease payments under the Amended and Restated Lease are as follows: (in thousands) Operating Lease Year Ending December 31, 2021 $ — 2022 1,681 2023 7,844 2024 8,080 2025 8,322 Thereafter 68,257 Total operating lease payments 94,184 Less: Amount representing interest (52,279) Present value of operating lease payments $ 41,905

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Data	The following is a summary of the quarterly results of the Company for the years ended December 31, 2020 and 2019 (unaudited, in thousands, except per share data): Three Months Ended 3/31/2020 6/30/2020 9/30/2020 12/31/2020 License and collaboration revenues $ 267 $ — $ 11,424 $ 1,707 Loss from operations (89,487) (84,862) (88,678) (106,336) Net loss (86,655) (82,859) (87,336) (101,087) Basic and diluted net loss per share $ (2.02) $ (1.89) $ (1.96) $ (2.08) Three Months Ended 3/31/2019 6/30/2019 9/30/2019 12/31/2019 License and collaboration revenues $ 1,244 $ 577 $ 988 $ 526 Loss from operations (42,758) (47,641) (57,059) (74,646) Net loss (40,912) (45,695) (54,273) (72,376) Basic and diluted net loss per share $ (1.17) $ (1.26) $ (1.38) $ (1.83)

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Estimated Useful Lives (Details)	12 Months Ended
	Dec. 31, 2020
Computer equipment
Property and equipment
Estimated useful lives	3 years
Office and other equipment
Property and equipment
Estimated useful lives	6 years
Laboratory equipment
Property and equipment
Estimated useful lives	6 years

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Asset impairment charges	$ 0	$ 0	$ 0

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Potentially Dilutive Shares (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities not included in the calculation of diluted net loss per share (shares)	11,714,118	12,634,061	13,413,024
Common stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities not included in the calculation of diluted net loss per share (shares)	2,503,294	2,403,055	1,781,388
Common stock warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities not included in the calculation of diluted net loss per share (shares)	9,210,824	10,231,006	11,631,636

Investments - Summary (Details)

Investments - Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 504,305	$ 368,146
Gross unrealized gains	265	375
Gross unrealized losses	(26)	(6)
Estimated fair value	504,544	368,515
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	130,814	160,065
Gross unrealized gains	160	233
Gross unrealized losses	(4)	(1)
Estimated fair value	$ 130,970	$ 160,297
Corporate debt securities | Maximum
Debt Securities, Available-for-sale [Line Items]
Maturity	2 years	2 years
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 240,725	$ 120,862
Gross unrealized gains	58	74
Gross unrealized losses	(18)	0
Estimated fair value	$ 240,765	$ 120,936
Commercial paper | Maximum
Debt Securities, Available-for-sale [Line Items]
Maturity	1 year	1 year
U.S. Agency bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 83,227	$ 50,745
Gross unrealized gains	37	41
Gross unrealized losses	(1)	(4)
Estimated fair value	$ 83,263	$ 50,782
U.S. Agency bonds | Maximum
Debt Securities, Available-for-sale [Line Items]
Maturity	2 years	2 years
U.S. Treasury bills
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 49,539	$ 36,474
Gross unrealized gains	10	27
Gross unrealized losses	(3)	(1)
Estimated fair value	$ 49,546	$ 36,500
U.S. Treasury bills | Maximum
Debt Securities, Available-for-sale [Line Items]
Maturity	2 years	2 years

Investments - Narrative (Detail

Investments - Narrative (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Allowance for credit losses on available-for-sale investments	$ 0	$ 0
ORIC
Schedule of Investments [Line Items]
Stock held in investment (shares)	588,235

Fair Value Measurements - Summa

Fair Value Measurements - Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash and cash equivalents	$ 885,562	$ 46,535
Investments	504,544	368,515
Total	1,405,735	415,050
U.S. Treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	49,546	36,500
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	130,970	160,297
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	240,765	120,936
U.S. Agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	83,263	50,782
Long-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	15,629
Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash and cash equivalents	20,398	662
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash and cash equivalents	865,164	45,873
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash and cash equivalents	885,562	46,535
Investments	49,546	36,500
Total	935,108	83,035
Level 1 | U.S. Treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	49,546	36,500
Level 1 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Level 1 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Level 1 | U.S. Agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Level 1 | Long-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0
Level 1 | Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash and cash equivalents	20,398	662
Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash and cash equivalents	865,164	45,873
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash and cash equivalents	0	0
Investments	454,998	332,015
Total	454,998	332,015
Level 2 | U.S. Treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Level 2 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	130,970	160,297
Level 2 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	240,765	120,936
Level 2 | U.S. Agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	83,263	50,782
Level 2 | Long-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0
Level 2 | Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash and cash equivalents	0	0
Level 2 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash and cash equivalents	0	$ 0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash and cash equivalents	0
Investments	0
Total	15,629
Level 3 | U.S. Treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0
Level 3 | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0
Level 3 | Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0
Level 3 | U.S. Agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0
Level 3 | Long-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	15,629
Level 3 | Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash and cash equivalents	0
Level 3 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash and cash equivalents	$ 0

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details)	Dec. 31, 2020
Volatility
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Measurement input of investment	0.92
Expected Term
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Measurement input of investment	1.1
Discount for Lack of Marketability
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Measurement input of investment	0.215

Fair Value Measurements - Chang

Fair Value Measurements - Changes in Estimated Fair Value of Assets with Unobservable Inputs (Details) - Level 3 $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 0
Additions	11,424
Change in fair value	4,205
Balance at end of period	$ 15,629

Other Current Assets and Othe_3

Other Current Assets and Other Long-Term Assets - Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid expenses	$ 8,158	$ 5,672
Deposits and other receivables	3,075	2,119
Interest receivables	2,304	1,566
Other current assets	$ 13,537	$ 9,357

Other Current Assets and Othe_4

Other Current Assets and Other Long-Term Assets - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Other long-term assets	$ 9,157	$ 5,435
Deposits paid for research and development	8,600	5,100
Letter of credit secured by restricted cash	600
Restricted cash included in other long-term assets	$ 620	$ 321	$ 0

Property and Equipment, Net - S

Property and Equipment, Net - Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property and equipment
Gross property and equipment	$ 9,479	$ 2,805
Less: Accumulated depreciation	(1,670)	(1,029)
Property and equipment, net	7,809	1,776
Computer equipment
Property and equipment
Gross property and equipment	201	201
Office and other equipment
Property and equipment
Gross property and equipment	329	329
Laboratory equipment
Property and equipment
Gross property and equipment	5,310	2,212
Leasehold improvements
Property and equipment
Gross property and equipment	$ 3,639	$ 63

Property and Equipment, Net - N

Property and Equipment, Net - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Depreciation	$ 641	$ 249	$ 175

Accounts Payable and Accrued _3

Accounts Payable and Accrued Liabilities - Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Accounts payable	$ 18,117	$ 16,367
Accrued clinical expense	32,240	21,290
Accrued development and other expense	7,151	2,510
Accrued compensation and benefits	13,964	7,915
Total accounts payable and accrued liabilities	$ 71,472	$ 48,082

Accounts Payable and Accrued _4

Accounts Payable and Accrued Liabilities - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Long-term liabilities	$ 1,962	$ 999

License and Collaboration Agr_3

License and Collaboration Agreements - Narrative (Details) - USD ($)	Aug. 03, 2020	Jan. 07, 2018	Oct. 31, 2014	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Expenses related to collaboration agreement				$ 299,349,000	$ 182,866,000	$ 93,872,000
BeiGene Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Up-front fee received		$ 10,000,000
Revenue from performance obligation expected to be earned		$ 123,000,000
Termination of contract, period after first commercial sale of product		10 years
Period required for notice of termination of contract		60 days
Revenue from performance obligation earned						500,000
Revenue from performance obligations satisfied during reporting period				172,000
BeiGene Agreement | Licenses of Intellectual Property
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from performance obligation earned				0	0	9,500,000
BeiGene Agreement | Manufacturing Supply Services
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from performance obligation earned				1,800,000	3,000,000	400,000
Revenue recognized				(2,000,000)	(3,300,000)	(500,000)
Revenue from performance obligations satisfied during reporting period				200,000	300,000	100,000
Cost sharing receivable				1,300,000	400,000
BeiGene Agreement | Milestone Payments
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from performance obligation expected to be earned				5,000,000
Revenue recognized				0	0	3,000,000
BeiGene Agreement | Royalties
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue				0	0	0
Pfizer Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Termination of contract, period after first commercial sale of product			10 years
Period required for notice of termination of contract			60 days
Expenses related to collaboration agreement				4,800,000	7,000,000	6,400,000
Pfizer Agreement | Research and Development Services
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Expenses related to collaboration agreement				$ 1,500,000	$ 6,000,000	4,100,000
Pfizer Agreement | Option Fee - Exclusive License to Develop and Commercialize MRTX1133
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Expenses related to collaboration agreement						$ 2,000,000
ORIC Pharmaceuticals Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Stock received as part of stock issuance and license agreements (shares)	588,235
License agreement, period of transfer restrictions	18 months
License agreement, period of agreement after first commercial sale	10 years
ORIC Pharmaceuticals Agreement | Level 3
Research and Development Arrangement, Contract to Perform for Others [Line Items]
License agreement, transaction price	$ 11,400,000

License and Collaboration Agr_4

License and Collaboration Agreements - Activity in Contract Liabilities (Details) - BeiGene Agreement $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Movement in Deferred Revenue [Roll Forward]
Opening balance at beginning of period	$ (172)
Revenue from performance obligations satisfied during reporting period	172
Closing balance at end of period	$ 0

Shareholders' Equity - Stock Re

Shareholders' Equity - Stock Reserved for Future Issuance (Details)	Dec. 31, 2020shares
Class of Stock [Line Items]
Common stock reserved for future issuance (shares)	14,961,265
Employee Stock Purchase Plan
Class of Stock [Line Items]
Common stock reserved for future issuance (shares)	128,436
Warrants to purchase common stock
Class of Stock [Line Items]
Common stock reserved for future issuance (shares)	8,229,632
Common stock options
Class of Stock [Line Items]
Common stock reserved for future issuance (shares)	4,429,489
Restricted stock units
Class of Stock [Line Items]
Common stock reserved for future issuance (shares)	465,260
Shares available for grant
Class of Stock [Line Items]
Common stock reserved for future issuance (shares)	1,708,448

Shareholders' Equity - Narrativ

Shareholders' Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended					12 Months Ended
	Oct. 31, 2020	Jan. 31, 2020	Jun. 30, 2019	Jan. 31, 2019	Jun. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jul. 02, 2020
Warrants - issued and outstanding
Stock issued in transaction (shares)	4,585,706	3,538,462	2,415,000	1,854,838
Sale price of common stock (USD per share)	$ 202	$ 97.50	$ 97	$ 62
Proceeds from sale of common stock	$ 879,600	$ 324,000	$ 219,900	$ 107,900
Common stock, par value (USD per share)						$ 0.001	$ 0.001		$ 0.001
Aggregate offering price									$ 200,000
Proceeds from disgorgement of shareholders' short-swing profits				$ 1,100		$ 41	$ 1,050	$ 0
Number of warrants exercised via cashless exercises (shares)						1,400,012	2,125,033	0
Net exercise of warrants (shares)						1,400,000	2,125,000
Exercise of warrants for cash (shares)						63,235
Warrant
Warrants - issued and outstanding
Stock issued in transaction (shares)					421,650
Sale price of common stock (USD per share)					$ 38.849

Shareholders' Equity - Warrants

Shareholders' Equity - Warrants Issued and Outstanding (Details) - $ / shares	Dec. 31, 2020	Jun. 11, 2018	Nov. 20, 2017	Jan. 11, 2017
Class of Stock [Line Items]
Number of warrants (shares)	8,229,632
Private Placement | January 11, 2017 Warrants
Class of Stock [Line Items]
Exercise price of warrants (USD per share)				$ 0.001
Number of warrants (shares)	3,733,218
Private Placement | November 20, 2017 Warrants
Class of Stock [Line Items]
Exercise price of warrants (USD per share)			$ 0.001
Number of warrants (shares)	4,137,999
Private Placement | June 11, 2018 Warrants
Class of Stock [Line Items]
Exercise price of warrants (USD per share)		$ 0.001
Number of warrants (shares)	358,415

Share-Based Compensation - Narr

Share-Based Compensation - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	May 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price required percentage of fair market value	100.00%
Award vesting period	4 years
Intrinsic value of options exercised	$ 121,500,000	$ 38,600,000	$ 18,800,000
Cash received from exercise of stock options	45,900,000	8,500,000	9,700,000
Fair value of options vested	52,100,000	20,400,000	8,200,000
Compensation cost not yet recognized	$ 61,400,000
Compensation cost not yet recognized, period for recognition	1 year 2 months 12 days
Capitalized costs	$ 0	$ 0	$ 0
Tax benefit from compensation expense	$ 0
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	4 years
Compensation cost not yet recognized	$ 31,400,000
Compensation cost not yet recognized, period for recognition	1 year 8 months 12 days
Fair value of equity instruments other than options that vested during the period	$ 100,000
2013 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional shares authorized (shares)		2,500,000
Authorized amount for issuance (shares)	1,500,000
Stock option contractual terms	10 years
Stock authorized for purchase (shares)				$ 25,000
Purchase discount (as a percent)				85.00%
Stock reserved for future issuance (shares)				300,000
Issuance of common stock from ESPP (shares)	171,564
Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Authorized amount for issuance (shares)	0
Stock option contractual terms	7 years
Inducement Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Option, Exercise Price Range, Shares Outstanding	246,728	417,343

Share-Based Compensation - Stoc

Share-Based Compensation - Stock Option Activity (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Balance at beginning of year (shares) | shares	5,287,977
Granted (shares) | shares	901,094
Exercised (shares) | shares	(1,318,963)
Canceled (shares) | shares	(440,619)
Balance at end of year (shares) | shares	4,429,489
Options exercisable at end of year (shares) | shares	2,374,753
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Balance at beginning of year (USD per share) | $ / shares	$ 44.10
Granted (USD per share) | $ / shares	115.83
Exercised (USD per share) | $ / shares	34.79
Canceled (USD per share) | $ / shares	70.64
Balance at end of year (USD per share) | $ / shares	58.82
Options exercisable at end of year (USD per share) | $ / shares	$ 33.62
Options outstanding, Weighted-Average Remaining Contractual Term (years)	7 years 3 months 18 days
Options exercisable, Weighted-Average Remaining Contractual Term (years)	6 years 2 months 12 days
Options outstanding, Aggregate Intrinsic Value | $	$ 712.5
Options exercisable, Aggregate Intrinsic Value | $	$ 441.7

Share-Based Compensation - RSU

Share-Based Compensation - RSU Activity (Details) - Restricted stock units $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)$ / sharesshares
Number of RSUs
Balance outstanding at beginning of period (shares) | shares	0
Granted (shares) | shares	487,569
Releases (shares) | shares	(938)
Canceled/forfeited (shares) | shares	(21,371)
Balance outstanding at end of period (shares) | shares	465,260
Weighted average grant date fair value
Balance outstanding at beginning of period (USD per share) | $ / shares	$ 0
Granted (USD per share) | $ / shares	113.91
Releases (USD per share) | $ / shares	113.20
Canceled/Forfeited (USD per share) | $ / shares	109
Balance outstanding at end of period (USD per share) | $ / shares	$ 114.14
Aggregate Intrinsic Value (millions) | $	$ 102.2

Share-Based Compensation - Assu

Share-Based Compensation - Assumptions Used in Fair Value Estimates (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Payment Arrangement [Abstract]
Risk-free interest rate (as a percent)	1.10%	2.20%	2.60%
Dividend yield (as a percent)	0.00%	0.00%	0.00%
Volatility factor (as a percent)	81.50%	82.10%	94.30%
Expected term (in years)	5 years 7 months 6 days	5 years 7 months 6 days	6 years
Weighted average estimated fair value per share (USD per share)	$ 77.92	$ 52.03	$ 24.39

Share-Based Compensation - Shar

Share-Based Compensation - Share-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 85,847	$ 55,537	$ 15,854
Research and development expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	48,044	31,024	7,232
General and administrative expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 37,803	$ 24,513	$ 8,622

Employee Benefit Plan - Narrati

Employee Benefit Plan - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Retirement Benefits [Abstract]
Proportion of employee contributions matched (as a percent)	4.00%
Maximum employer contribution per employee	$ 2,500
Matching contribution amount	$ 1,300,000	$ 200,000	$ 100,000

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details)	12 Months Ended
	Dec. 31, 2020USD ($)ongoing_tax_audit	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Income Tax [Line Items]
Income tax expense (benefit)	$ 0	$ 0	$ 0
Amount of increase in valuation allowance	(111,000,000)
Accrual for interest or penalties on tax matters	$ 0	0	0
Number of ongoing tax audits | ongoing_tax_audit	0
Federal Tax Authority
Income Tax [Line Items]
Income tax expense (benefit)	$ 0	0	0
Canadian Federal
Income Tax [Line Items]
Scientific research and experimental development expenses deductible for income tax purposes	19,900,000	19,900,000	19,900,000
Net operating loss carry forwards (NOLs)	79,294,000
Canadian Provincial
Income Tax [Line Items]
Scientific research and experimental development expenses deductible for income tax purposes	21,600,000	$ 21,600,000	$ 21,600,000
Net operating loss carry forwards (NOLs)	78,673,000
US Federal
Income Tax [Line Items]
Net operating loss carry forwards (NOLs)	702,882,000
US Federal | Research Tax Credit Carryforward
Income Tax [Line Items]
Tax credit carryforward	29,700,000
US State
Income Tax [Line Items]
Net operating loss carry forwards (NOLs)	274,324,000
US State | Research Tax Credit Carryforward
Income Tax [Line Items]
Tax credit carryforward	$ 11,300,000

Income Taxes - Effective Income

Income Taxes - Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Net loss before tax	$ (357,937)	$ (213,256)	$ (98,418)
Statutory combined U.S. federal and state tax rate (as a percent)	21.00%	21.00%	21.00%
Statutory federal and state taxes	$ (75,167)	$ (44,784)	$ (20,668)
State income taxes, net of federal benefit	(13,490)	0	0
Effect of change in valuation allowance	110,985	52,719	25,959
Non-deductible share-based compensation	2,724	1,810	884
Tax deductions for share-based compensation	(17,991)	(6,917)	(2,924)
Tax credits	(15,672)	(8,621)	(5,130)
Uncertain tax positions	3,857	2,143	1,283
Return to provision and other true-ups	25	(60)	375
Non-deductible officers' compensation	4,697	3,527	179
Other differences	32	183	42
Income tax benefit	$ 0	$ 0	$ 0

Income Taxes - Net Deferred Inc

Income Taxes - Net Deferred Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Tangible and intangible depreciable assets	$ 32,180	$ 6,978
Stock compensation	19,183	12,321
Provisions	2,510	934
Other, net	0	35
Lease liability	8,800	137
Net operating loss carry forwards	182,536	116,345
Capital loss carryforward	114	178
Canada scientific research and experimental development expenditures	5,471	5,471
U.S. research and development tax credits	28,834	17,080
Total gross deferred tax assets	279,628	159,479
Less valuation allowance	(270,368)	(159,357)
Net deferred tax assets	9,260	122
Deferred tax liabilities:
Right-of-use asset	(8,377)	(122)
Non-current investment	(883)	0
Net deferred income taxes	$ 0	$ 0

Income Taxes - Net Operating Lo

Income Taxes - Net Operating Loss Expirations (Details) $ in Thousands	Dec. 31, 2020USD ($)
US Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	$ 702,882
US Federal | 2030
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	0
US Federal | 2031
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	0
US Federal | 2032
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	0
US Federal | 2033
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	2,225
US Federal | 2034
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	7,276
US Federal | 2035
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	53,359
US Federal | 2036
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	23,379
US Federal | 2037
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	65,509
US Federal | 2038
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	0
US Federal | 2039
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	0
US Federal | 2040
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	0
US Federal | Does not expire
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	551,134
US State
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	274,324
US State | 2030
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	0
US State | 2031
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	0
US State | 2032
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	0
US State | 2033
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	2,232
US State | 2034
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	22,162
US State | 2035
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	52,950
US State | 2036
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	0
US State | 2037
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	0
US State | 2038
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	3,817
US State | 2039
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	0
US State | 2040
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	193,163
US State | Does not expire
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	0
Canadian Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	79,294
Canadian Federal | 2030
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	4,830
Canadian Federal | 2031
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	7,059
Canadian Federal | 2032
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	13,308
Canadian Federal | 2033
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	18,623
Canadian Federal | 2034
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	32,401
Canadian Federal | 2035
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	1,084
Canadian Federal | 2036
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	777
Canadian Federal | 2037
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	697
Canadian Federal | 2038
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	0
Canadian Federal | 2039
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	242
Canadian Federal | 2040
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	273
Canadian Federal | Does not expire
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	0
Canadian Provincial
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	78,673
Canadian Provincial | 2030
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	4,907
Canadian Provincial | 2031
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	7,066
Canadian Provincial | 2032
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	12,433
Canadian Provincial | 2033
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	19,385
Canadian Provincial | 2034
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	31,809
Canadian Provincial | 2035
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	1,084
Canadian Provincial | 2036
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	777
Canadian Provincial | 2037
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	697
Canadian Provincial | 2038
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	0
Canadian Provincial | 2039
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	242
Canadian Provincial | 2040
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	273
Canadian Provincial | Does not expire
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	$ 0

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Tax Positions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Canadian Federal
Reconciliation of the beginning and ending gross amounts of unrecognized tax positions
Unrecognized tax positions, beginning of year	$ 4,268	$ 2,617	$ 1,693
Gross increase — current period tax positions	3,126	1,651	924
Gross decrease — prior period tax positions	0	0	0
Gross increase — prior period tax positions	0	0	0
Expiration of statute of limitations	0	0	0
Unrecognized tax positions, end of year	7,394	4,268	2,617
Canadian Provincial
Reconciliation of the beginning and ending gross amounts of unrecognized tax positions
Unrecognized tax positions, beginning of year	8,648	8,010	7,556
Gross increase — current period tax positions	1,004	638	454
Gross decrease — prior period tax positions	0	0	0
Gross increase — prior period tax positions	0	0	0
Expiration of statute of limitations	0	0	0
Unrecognized tax positions, end of year	$ 9,652	$ 8,648	$ 8,010

Investment Tax Credits - Summar

Investment Tax Credits - Summary (Details) - Canadian Federal $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
INVESTMENT TAX CREDITS
Non-refundable ITCs related to scientific research and development expenditures	$ 3,920
2030
INVESTMENT TAX CREDITS
Non-refundable ITCs related to scientific research and development expenditures	764
2031
INVESTMENT TAX CREDITS
Non-refundable ITCs related to scientific research and development expenditures	1,000
2032
INVESTMENT TAX CREDITS
Non-refundable ITCs related to scientific research and development expenditures	1,125
2033
INVESTMENT TAX CREDITS
Non-refundable ITCs related to scientific research and development expenditures	$ 1,031

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
Operating right-of-use asset	$ 39,890,000	$ 582,000
Operating lease liability		700,000
Operating lease cost		$ 400,000
Current Headquarters
Lessee, Lease, Description [Line Items]
Discount rate (as a percent)	12.00%
Operating right-of-use asset	$ 39,900,000
Operating lease liability	41,905,000
Operating lease expense	$ 300,000
Building | Future Headquarters
Lessee, Lease, Description [Line Items]
Lease term	12 years
Future monthly rent expense	$ 600,000
Annual rent increase (as a percent)	3.00%
Building | Current Headquarters
Lessee, Lease, Description [Line Items]
Annual rent increase (as a percent)	3.00%
Monthly rent expense	$ 24,100

Commitments and Contingencies_2

Commitments and Contingencies - Future Minimum Lease Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
Present value of operating lease payments		$ 700
Current Headquarters
Lessee, Lease, Description [Line Items]
2021	$ 0
2022	1,681
2023	7,844
2024	8,080
2025	8,322
Thereafter	68,257
Total operating lease payments	94,184
Less: Amount representing interest	(52,279)
Present value of operating lease payments	$ 41,905

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Unaudited) - Summary (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
License and collaboration revenues	$ 1,707	$ 11,424	$ 0	$ 267	$ 526	$ 988	$ 577	$ 1,244	$ 13,398	$ 3,335	$ 12,926
Loss from operations	(106,336)	(88,678)	(84,862)	(89,487)	(74,646)	(57,059)	(47,641)	(42,758)	(369,363)	(222,104)	(102,627)
Net loss	$ (101,087)	$ (87,336)	$ (82,859)	$ (86,655)	$ (72,376)	$ (54,273)	$ (45,695)	$ (40,912)	$ (357,937)	$ (213,256)	$ (98,418)
Basic and diluted net loss per share (USD per share)	$ (2.08)	$ (1.96)	$ (1.89)	$ (2.02)	$ (1.83)	$ (1.38)	$ (1.26)	$ (1.17)	$ (7.96)	$ (5.69)	$ (3.19)

Uncategorized Items - mrtx-2020

Label	Element	Value
Pfizer Discovery and Collaboration Agreement [Member] | Milestone Payments - Initiation of First Phase 2 Trial for MRTX849 [Member]
Research and Development Expense	us-gaap_ResearchAndDevelopmentExpense	$ 3,000,000
Pfizer Discovery and Collaboration Agreement [Member] | Development Milestone Payments [Member]
Research and Development Arrangement, Discovery and Collaboration Agreement, Compensation Expected to be Paid	mrtx_ResearchandDevelopmentArrangementDiscoveryandCollaborationAgreementCompensationExpectedtobePaid	9,300,000
Research and Development Arrangement, Discovery and Collaboration Agreement, Milestone Payments Paid	mrtx_ResearchandDevelopmentArrangementDiscoveryandCollaborationAgreementMilestonePaymentsPaid	4,500,000
Pfizer Discovery and Collaboration Agreement [Member] | Milestone Payments - Initiation of First Regulatory Toxicology Study for MRTX1133 [Member]
Research and Development Expense	us-gaap_ResearchAndDevelopmentExpense	300,000
Research and Development Expense	us-gaap_ResearchAndDevelopmentExpense	300,000
Pfizer Discovery and Collaboration Agreement [Member] | Milestone Payments - Initiation of First Phase 1 Trial for MRTX849 [Member]
Research and Development Expense	us-gaap_ResearchAndDevelopmentExpense	1,000,000
Pfizer Discovery and Collaboration Agreement [Member] | Sales Milestone Payments [Member]
Research and Development Arrangement, Discovery and Collaboration Agreement, Compensation Expected to be Paid	mrtx_ResearchandDevelopmentArrangementDiscoveryandCollaborationAgreementCompensationExpectedtobePaid	$ 337,000,000