Mirati Therapeutics (MRTX) 10-K 26 Feb 20 2019 FY Annual report Financial data

Cover Page

Cover Page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Feb. 20, 2020	Jun. 28, 2019
Cover page.
Entity Central Index Key	0001576263
Amendment Flag	false
Document Fiscal Year Focus	2019
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, 2019
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
Entity Shell Company	false
Entity Public Float			$ 2,907
Entity Common Stock, Shares Outstanding		43,480,402

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 46,535	$ 32,694
Short-term investments	368,515	190,096
Other current assets	9,357	3,870
Total current assets	424,407	226,660
Property and equipment, net	1,776	473
Other long-term assets	6,017	1,321
Total assets	432,200	228,454
Current liabilities
Accounts payable and accrued liabilities	48,082	25,775
Deferred revenue and other current liabilities	824	371
Total current liabilities	48,906	26,146
Deferred revenue and other liabilities	999	732
Total liabilities	49,905	26,878
Stockholders' equity
Preferred stock, $0.001 par value, 10,000,000 shares authorized; none issued and outstanding at both December 31, 2019 and December 31, 2018	0	0
Common stock, $0.001 par value; 100,000,000 authorized; 39,517,329 and 32,538,857 issued and outstanding at December 31, 2019 and December 31, 2018, respectively	40	33
Additional paid-in capital	1,144,667	751,109
Accumulated other comprehensive income	9,889	9,479
Accumulated deficit	(772,301)	(559,045)
Total stockholders' equity	382,295	201,576
Total liabilities and stockholders' equity	$ 432,200	$ 228,454

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	39,517,329	32,538,857
Common stock, shares outstanding	39,517,329	32,538,857

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue
Total revenue	$ 3,335	$ 12,926	$ 0
Expenses
Research and development	182,866	93,872	58,085
General and administrative	42,573	21,681	13,450
Total operating expenses	225,439	115,553	71,535
Loss from operations	(222,104)	(102,627)	(71,535)
Other income, net	8,848	4,209	1,105
Net loss	(213,256)	(98,418)	(70,430)
Unrealized gain (loss) on available-for-sale investments	(410)	0	54
Comprehensive loss	$ (212,846)	$ (98,418)	$ (70,484)
Basic and diluted net loss per share (USD per share)	$ (5.69)	$ (3.19)	$ (2.78)
Weighted average common shares outstanding, basic and diluted (shares)	37,467,505	30,897,717	25,290,222

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
Beginning balance (shares) at Dec. 31, 2016		19,937,095
Beginning balance at Dec. 31, 2016	$ 48,309	$ 20	$ 428,507	$ 9,533	$ (389,751)
Increase (Decrease) in Stockholders' Equity
Net loss	(70,430)				(70,430)
Issuance of common stock and warrants, net of issuance costs (shares)		7,941,688
Issuance of common stock and warrants, net of issuance costs	153,530	$ 8	153,522
Share-based compensation expense	6,786		6,786
Issuance of common stock from ESPP (shares)		59,976
Issuance of common stock from ESPP	144		144
Exercise of options for cash (shares)		45,573
Exercise of options for cash	$ 399		399
Exercise of warrants for cash (shares)	585,729	585,729
Exercise of warrants for cash	$ 4,604	$ 1	4,603
Net exercise of warrants (shares)	638,554	52,825
Unrealized gain (loss) on investments	$ (54)			(54)
Ending balance (shares) at Dec. 31, 2017		28,622,886
Ending balance 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
Exercise of options for cash (shares)		731,935
Exercise of options for cash	9,747	$ 1	9,746
Unrealized gain (loss) on investments	$ 0
Ending balance (shares) at Dec. 31, 2018	32,538,857	32,538,857
Ending balance 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
Exercise of options for cash (shares)	569,146	569,146
Exercise of options for cash	$ 8,473	$ 1	8,472
Adjustments to Additional Paid-in-Capital, Disgorgement of Stockholders' Short-Swing Profits	$ 1,050		1,050
Net exercise of warrants (shares)	2,125,000	2,125,000
Net exercise of warrants		$ 2	(2)
Unrealized gain (loss) on investments	$ 410			410
Ending balance (shares) at Dec. 31, 2019	39,517,329	39,517,329
Ending balance at Dec. 31, 2019	$ 382,295	$ 40	$ 1,144,667	$ 9,889	$ (772,301)

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Operating activities
Net loss	$ (213,256)	$ (98,418)		$ (70,430)
Non-cash adjustments reconciling net loss to operating cash flows
Depreciation of property and equipment	249	175		184
Accretion of discount on investments	(3,421)	(1,320)		(266)
Share-based compensation expense	55,537	15,854		6,786
Changes in operating assets and liabilities:
Other current assets	(5,487)	1,052		(3,409)
Other long-term assets	(4,375)	(359)		3,606
Accounts payable, accrued liabilities, deferred revenue and other liabilities	23,027	12,920		(1,177)
Cash flows used in operating activities	(147,726)	(70,096)		(64,706)
Investing activities:
Purchases of short-term investments	(530,228)	(255,795)		(100,558)
Sales and maturities of short-term investments	355,640	110,152		91,988
Purchases of property and equipment	(1,552)	(122)		(81)
Cash flows used in investing activities	(176,140)	(145,765)		(8,651)
Financing activities:
Proceeds from issuance of common stock and warrants, net of issuance costs	327,830	130,663	[1]	153,530	[2]
Proceeds from exercise of common stock options and warrants	8,473	9,747		5,003
Proceeds from disgorgement of stockholders' short-swing profits	1,050	0		0
Proceeds from issuances under employee stock purchase plan	675	442		144
Cash flows provided by financing activities	338,028	140,852		158,677
Increase (decrease) in cash, cash equivalents and restricted cash	14,162	(75,009)		85,320
Cash, cash equivalents and restricted cash, beginning of year	32,694	107,703		22,383
Cash, cash equivalents and restricted cash, end of year	46,856	32,694		107,703
Reconciliation of cash, cash equivalents and restricted cash, end of period:
Total cash, cash equivalents and restricted cash	$ 32,694	$ 32,694		$ 22,383
[1]	Proceeds in 2018 include 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.
[2]	Proceeds in 2017 include warrants to purchase up to 4,137,999 shares of the Company's common stock at a public offering price of $12.999 per warrant, net of issuance costs, and warrants to purchase up to 7,258,263 shares of the Company's common stock at a public offering price of $5.599 per warrant, net of issuance costs.

CONSOLIDATED STATEMENTS OF CA_2

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - $ / shares	1 Months Ended
	Jun. 30, 2019	Jan. 31, 2019	Jun. 30, 2018	Nov. 30, 2017	Jan. 31, 2017
Stock issued in transaction (shares)	2,415,000	1,854,838	3,162,500
Sale price of common stock (USD per share)	$ 97	$ 62	$ 38.85
Warrant
Stock issued in transaction (shares)			421,650	4,137,999	7,258,263
Sale price of common stock (USD per share)			$ 38.849	$ 12.999	$ 5.599

Description of Business

Description of Business	12 Months Ended
	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of Business	Description of Business Mirati 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 in one business segment, 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, 2019
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 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 stockholders' 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 a collaboration and license agreement 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 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 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, 2019 , 2018 and 2017 . Share-Based Compensation The Company measures and recognizes compensation expense for share-based payments based on estimated fair value, 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 is recognized using the graded accelerated vesting method. Research and Development Expenses Research and development expenditures are charged to 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 our early discovery efforts. General and Administrative Expenses General and administrative expenses consist primarily of salaries and related benefits, including share-based compensation, related to our executive, finance, business development, legal, human resources and support functions. Other general and administrative expenses include professional fees for auditing, tax, consulting and patent-related services, rent and utilities and insurance. 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 common 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, 2019 2018 2017 Common stock options 2,403,055 1,781,388 38,675 Common stock warrants 10,231,006 11,631,636 7,534,576 Total 12,634,061 13,413,024 7,573,251

Recently Issued and Recently Ad

Recently Issued and Recently Adopted Accounting Pronouncements	12 Months Ended
	Dec. 31, 2019
New Accounting Pronouncements and Changes in Accounting Principles [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 February 2016, the FASB issued Accounting Standards Update ("ASU") 2016-02, Leases (Topic 842) in order to increase transparency and comparability among organizations by recognizing lease assets and lease liabilities on the balance sheet for those leases classified as operating leases under previous GAAP. ASU 2016-02 requires a lessee to recognize a liability for lease payments (the lease liability) and a right-of-use asset (representing its right to use the underlying asset for the lease term) on the balance sheet. ASU 2016-02 is effective for fiscal years beginning after December 15, 2018 (including interim periods within those periods) using a modified retrospective approach. In July 2018, the FASB issued ASU 2018-11, Leases (Topic 842): Targeted Improvements , which provides entities an optional transition method to apply the new guidance as of the adoption date, rather than as of the earliest period presented. In transition, entities may also elect a package of practical expedients that must be applied in its entirety to all leases commencing before the effective date, unless the lease was modified, to not reassess (a) the existence of a lease, (b) lease classification or (c) determination of initial direct costs, which effectively allows entities to carryforward accounting conclusions under previous U.S. GAAP. The Company adopted ASU 2016-02, using the optional transition method and electing the package of practical expedients described above on January 1, 2019. Due to the adoption, the Company recognized a new lease liability on the Company's consolidated balance sheet for its operating lease of office and lab space of $367,000 on January 1, 2019, with a corresponding right-of-use asset of the same amount based on the present value of the remaining minimum rental payments. See Note 15 for further discussion. In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260); Distinguishing Liabilities from Equity (Topic 480); Derivatives and Hedging (Topic 815): (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Non-controlling Interests with a Scope Exception . This ASU allows companies to exclude a down round feature when determining whether a financial instrument (or embedded conversion feature) is considered indexed to the entity’s own stock. As a result, financial instruments (or embedded conversion features) with down round features may no longer be required to be classified as liabilities. A company will recognize the value of a down round feature only when it is triggered and the strike price has been adjusted downward. For equity-classified freestanding financial instruments, such as warrants, an entity will treat the value of the effect of the down round, when triggered, as a dividend and a reduction of income available to common shareholders in computing basic earnings per share. For convertible instruments with embedded conversion features containing down round provisions, entities will recognize the value of the down round as a beneficial conversion discount to be amortized to earnings. Effective January 1, 2019, the Company adopted the provisions of ASU 2017-11. The adoption did not have a material impact on the Company's consolidated financial statements or related financial statement disclosures. In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 . The amendments provide guidance on whether certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606. It also specifically (i) addresses when the participant should be considered a customer in the content of a unit of account, (ii) adds unit of account guidance in ASC 808 to align with guidance with ASC 606, and (iii) precludes presenting revenue from a collaborative arrangement together with revenue recognized under ASC 606 if the collaborative arrangement participant is not a customer. The guidance in ASU 2018-18 is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. Early adoption is permitted and should be applied retrospectively. The Company elected to early adopt this guidance effective January 1, 2019. The adoption had no impact on the Company's consolidated financial statements. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230) Restricted Cash . This ASU requires that an entity reconcile and explain the period-over-period change in total cash, cash equivalents and restricted cash within its statements of cash flows. The Company adopted ASU 2016-18 on January 1, 2019. The adoption of ASU 2016-18 did not have a material impact on the Company's consolidated financial statements or related financial statement disclosures. Recently Issued Accounting Pronouncements In June 2016, the FASB issued 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 will be required to use a new forward-looking expected loss model that generally will result in the earlier recognition of allowances for losses. For available-for-sale debt securities with unrealized losses, the losses will be 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, with early adoption permitted only as of annual reporting periods beginning after December 15, 2018. The Company does not anticipate that the adoption of ASU 2016-13 will 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. The Company does not anticipate that the adoption of ASU 2018-13 will have a material impact on the Company's consolidated financial statements or related financial statement disclosures.

Investments

Investments	12 Months Ended
	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Investments	Investments The following tables summarize the Company's short-term investments (in thousands): 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 As of December 31, 2018 Maturity Amortized cost Gross unrealized gains Gross unrealized losses Estimated fair value Corporate debt securities 1 year or less $ 111,933 $ 26 $ (43 ) $ 111,916 Commercial paper 1 year or less 74,433 — (24 ) 74,409 U.S. Treasury bills 1 year or less 3,771 — — 3,771 $ 190,137 $ 26 $ (67 ) $ 190,096 Unrealized gains and losses on available-for-sale securities are included as a component of comprehensive loss. At December 31, 2019

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2019
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 or 2 within the fair value hierarchy as described in the accounting standards for fair value measurements. The Company has no financial assets or liabilities recorded at fair value which have been classified as Level 3. 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, 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 December 31, 2018 Total Level 1 Level 2 Assets Cash and cash equivalents: Cash $ 3,731 $ 3,731 $ — Money market funds 28,963 28,963 — Total cash and cash equivalents 32,694 32,694 — Short-term investments: U.S. Treasury Bills 3,771 3,771 — Corporate debt securities 111,916 — 111,916 Commercial paper 74,409 — 74,409 Total short-term investments 190,096 3,771 186,325 Total $ 222,790 $ 36,465 $ 186,325 The Company’s investments in Level 1 assets are valued based on publicly available quoted market prices for identical securities as of December 31, 2019 and 2018 . 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. There were no transfers between fair value measurement levels for the years ended December 31, 2019 and 2018

Other Current Assets and Other

Other Current Assets and Other Long-Term Assets	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 Prepaid expenses $ 5,672 $ 1,261 Deposits and other receivables 2,119 1,841 Interest receivables 1,566 768 $ 9,357 $ 3,870 The other long-term assets balance as of December 31, 2019 consists of $5.1 million in deposits paid in conjunction with the Company's research and development activities, $0.6 million for an operating right-of-use asset for the Company's corporate headquarters, and $0.3 million for a security deposit in connection with the lease of the Company's future corporate headquarters. As of December 31, 2018 , the other long-term assets balance consisted of $1.3 million in deposits paid in conjunction with the Company's research and development activities.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 Computer equipment $ 201 $ 201 Office and other equipment 329 260 Laboratory equipment 2,212 729 Leasehold improvements 63 63 Gross property and equipment 2,805 1,253 Less: Accumulated depreciation (1,029 ) (780 ) Property and equipment, net $ 1,776 $ 473 The Company incurred depreciation expense of $0.2 million during the years ended December 31, 2019 , 2018 and 2017

Accounts Payable and Accrued Li

Accounts Payable and Accrued Liabilities	12 Months Ended
	Dec. 31, 2019
Payables and Accruals [Abstract]
Accounts Payable and Accrued Liabilities	Accounts Payable and Accrued Liabilities Accounts payable and accrued liabilities consisted of the following (in thousands): December 31, 2019 2018 Accounts payable $ 16,367 $ 8,531 Accrued clinical expense 21,290 10,154 Accrued development and other expense 2,510 1,243 Accrued compensation and benefits 7,915 5,847 $ 48,082 $ 25,775 The long-term liabilities balance of $1.0 million as of December 31, 2019 consisted primarily of clinical trial-related liabilities. As of December 31, 2018 , the long-term liabilities of $0.7 million consisted of $0.1 million in deferred revenue and $0.6 million

BeiGene Agreement

BeiGene Agreement	12 Months Ended
	Dec. 31, 2019
Research and Development [Abstract]
BeiGene Agreement	9. BeiGene Agreement Terms of Agreement On January 7, 2018, the Company and BeiGene Ltd, ("BeiGene") entered into a Collaboration and License Agreement (the “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 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 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 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 Agreement also provides that the Company will supply BeiGene with sitravatinib for use in BeiGene’s development activities in the Licensed Territory. The 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 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 Agreement at any time by providing 60 days prior written notice to the Company. Either party may terminate the 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 Agreement upon written notice to BeiGene under specified circumstances if BeiGene challenges the licensed patent rights. Revenue Recognition The Company evaluated the Agreement under Topic 606. In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under the 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 is 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 year ended December 31, 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 will continue into 2020. The Company recognized $3.3 million as license and collaboration revenues for this performance obligation for 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. At December 31, 2019 , $0.4 million of cost-sharing receivable from BeiGene has been recorded in other current assets on the consolidated balance sheets. Milestone Payments. The Company is entitled to development milestones under the agreement. The Company did not recognize revenue associated with development milestones during the year ended December 31, 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, 2019 , 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 year ended December 31, 2019 . The following table presents a summary of the activity in the Company's contract liabilities during the year ended December 31, 2019 (in thousands): Opening balance, January 1, 2019 $ (481 ) Revenue from performance obligations satisfied during reporting period 309 Closing Balance, December 31, 2019 $ (172 ) The closing balance represents deferred revenue and is classified within current liabilities at December 31, 2019

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Stockholders' Equity	Stockholders' Equity Common Stock The following shares were reserved for future issuance: December 31, 2019 Common stock options outstanding and available for future grant 7,923,098 Warrants to purchase common stock 9,692,879 Employee Stock Purchase Plan 142,872 17,758,849 Sale of Common Stock 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 . In June 2018, the Company sold 3,162,500 shares of its common stock at a public offering price of $38.85 per share and sold warrants to purchase up to 421,650 shares of its common stock at a public offering price of $38.849 per warrant. After deducting underwriter discounts and offering expenses, the Company received net proceeds from the transaction of $130.7 million . The public offering price for the warrants was equal to the public offering price of the common stock, less the $0.001 per share exercise price of each warrant. The warrants were recorded as a component of stockholders’ equity within additional paid-in capital. Per their terms, the outstanding warrants to purchase shares of common stock may not be exercised if certain holders' ownership of the Company’s common stock would exceed 9.99% for certain holders, or 4.99% percent for other holders, following such exercise. 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. The Company recognized these proceeds as a capital contribution from st ockholders and reflected a corresponding increase to additional paid-in capital. Warrants As of December 31, 2019 , 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 5,133,230 November 20, 2017 None $ 0.001 4,137,999 June 11, 2018 None $ 0.001 421,650 9,692,879 During the year ended December 31, 2019 , warrants for 2,125,033 shares of the Company's common stock were exercised via cashless exercises, resulting in the issuance of 2,125,000 shares of common stock. During the year ended December 31, 2018 , no warrants were exercised. During the year ended December 31, 2017 , warrants for 52,825 shares of the Company's common stock were exercised via cashless exercises and 585,729 shares were exercised for cash, generating proceeds of $4.6 million and resulting in the issuance of 638,554 shares of common stock.

Share-Based Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2019
Share-based Payment Arrangement [Abstract]
Share-Based Compensation	Share-Based Compensation Equity Incentive Plan The Company has in place 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 our stockholders in connection with the Arrangement. The Company's Board of Directors and stockholders 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, 2019 , there were approximately 2.2 million stock options available to be issued 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, 2019 , no grants were issued from the Inducement Plan. As of December 31, 2019 , share-based compensation awards under both the Stock Option Plan and the 2013 Plan consist of incentive and non-qualified stock options. 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 . The following table summarizes the Company's stock option activity and related information for the year ended December 31, 2019 : Number of options Weighted average exercise price Weighted-Average Remaining Contractual Term (years) Aggregate Intrinsic Value (millions) Balance outstanding as of December 31, 2018 3,683,786 $ 19.59 Granted 2,310,994 $ 76.22 Exercised (569,146 ) $ 14.89 Canceled/forfeited (137,657 ) $ 48.31 Balance outstanding as of December 31, 2019 5,287,977 $ 44.10 7.7 $ 448.2 Options exercisable at December 31, 2019 2,339,904 $ 21.11 6.1 $ 252.1 The total intrinsic value of stock options exercised was $38.6 million , $18.8 million and $0.3 million for the years ended December 31, 2019 , 2018 , and 2017 , respectively. The Company received total cash of $8.5 million , $9.7 million and $0.4 million for the exercise of options for the years ended December 31, 2019 , 2018 and 2017 , respectively. The total fair value of options vested during the years ended December 31, 2019 , 2018 and 2017 was $20.4 million , $8.2 million and $10.6 million , respectively. Upon option exercise, the Company issues new shares of its common stock. Total share-based compensation expense by statement of operations and comprehensive loss classification is presented below (in thousands): Year ended December 31, 2019 2018 2017 Research and development expense $ 31,024 $ 7,232 $ 3,192 General and administrative expense 24,513 8,622 3,594 $ 55,537 $ 15,854 $ 6,786 For the years ended December 31, 2019 , 2018 and 2017 , no share-based compensation expense was capitalized and there were no recognized tax benefits associated with the share-based compensation charge. 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 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, 2019 2018 2017 Risk-free interest rate 2.2% 2.6% 2.1% Dividend yield —% —% —% Volatility factor 82.1% 94.3% 96.0% Expected term (in years) 5.6 6.0 6.0 Weighted average estimated fair value per share $52.03 $24.39 $4.17 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, 2019 related to non-vested option awards was $77.4 million which will be recognized over a weighted-average period of 1.3 years. 2013 Employee Stock Purchase Plan In May 2013, the Company's Board of Directors adopted the ESPP. The ESPP was approved by the Company's stockholders 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, 2019 , 157,128 shares have been issued out of the plan.

Employee Benefit Plan

Employee Benefit Plan	12 Months Ended
	Dec. 31, 2019
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. The Company matches up to 4% of an employee's contributions, subject to a limit of $2,500 per year. Expense associated with the Company's matching contribution totaled $0.2 million for the year ended December 31, 2019 , and $0.1 million for the years ended December 31, 2018 and 2017

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
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, 2019 , 2018 and 2017 . The differences between the effective income tax rate and the statutory tax rates during the years ended 2019 , 2018 and 2017 are as follows (in thousands): Year Ended December 31, 2019 2018 2017 Net loss before tax $ (213,256 ) $ (98,418 ) $ (70,430 ) Statutory combined U.S. federal and state tax rate 21.00 % 21.00 % 34.00 % Statutory federal and state taxes (44,784 ) (20,668 ) (23,946 ) Increase (decrease) in taxes recoverable resulting from: Effect of change in valuation allowance 52,719 25,959 (4,154 ) Non-deductible share-based compensation 1,810 884 695 Tax deductions for share-based compensation (6,917 ) (2,924 ) (80 ) Tax credits (8,621 ) (5,130 ) (2,563 ) Write off of Methylgene US Inc. net operating loss — — 307 Change in tax rate — — 303 Tax Cuts and Jobs Act — — 28,569 Uncertain tax positions 2,143 1,283 646 Return to provision and other true-ups (60 ) 375 394 Non-deductible officers' compensation 3,527 179 — Other differences 183 42 (171 ) Income tax benefit $ — $ — $ — Deferred Tax The following table summarizes the significant components of our deferred tax assets (in thousands): December 31, 2019 2018 Deferred tax assets: Tangible and intangible depreciable assets $ 6,978 $ 8,123 Stock compensation 12,321 6,515 Provisions 934 1,020 Other, net 35 — Lease liability 137 — Net operating loss carry forwards 116,345 74,721 Capital loss carryforward 178 178 Canada scientific research and experimental development expenditures 5,471 5,467 U.S. research and development tax credits 17,080 10,613 Total gross deferred tax assets 159,479 106,637 Less valuation allowance (159,357 ) (106,637 ) Net deferred tax assets $ 122 $ — Deferred tax liabilities: Right-of-use Asset $ (122 ) $ — Net deferred income taxes $ — $ — Total valuation allowance increased by $52.7 million for the year ended December 31, 2019 . The Company has established a full valuation allowance against its net deferred tax assets as of December 31, 2019 due to the uncertainty surrounding the realization of such assets as evidenced by the cumulative losses from operations through December 31, 2019 . 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, 2019 , 2018 and 2017 and for provincial income tax purposes amounted to $21.6 million at December 31, 2019 , 2018 and 2017 . As operations in Canada ceased during 2014, no expenditures were incurred for the years ended December 31, 2019 , 2018 and 2017 . 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, 2019 of $17.2 million and $7.2 million , respectively. The federal credits will begin to expire in 2033 unless utilized and the state credits have an indefinite life. At December 31, 2019 , the Company's net operating loss carry forwards ("NOLs") for U.S. federal and state income taxes were $452.5 million and $81.2 million , respectively and the Company's NOLs for Canadian federal and provincial income tax purposes were $79.1 million and $78.4 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 — — 274 274 Does not expire 300,763 — — — $ 452,511 $ 81,161 $ 79,053 $ 78,432 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, 2019. 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, 2019 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, 2015 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, 2019 2018 2017 2019 2018 2017 Unrecognized tax positions, beginning of year $ 2,617 $ 1,693 $ 1,095 $ 8,010 $ 7,556 $ 7,333 Gross increase — current period tax positions 1,651 924 588 638 454 227 Gross decrease — prior period tax positions — — — — — (3 ) Gross increase — prior period tax positions — — 11 — — — Expiration of statute of limitations — — (1 ) — — (1 ) Unrecognized tax positions, end of year $ 4,268 $ 2,617 $ 1,693 $ 8,648 $ 8,010 $ 7,556 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 expect any significant increases or decreases to the Company's unrecognized tax positions within the next 12 months. The Company recognizes interest and penalties related to unrecognized tax benefits in income tax expense. The Company had no accrual for interest or penalties on tax matters as of December 31, 2019 , 2018 and 2017 , and the Company had no ongoing tax audits as of December 31, 2019

Investment Tax Credits

Investment Tax Credits	12 Months Ended
	Dec. 31, 2019
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, 2019 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 1,018 $ 3,907

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies 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. On March 23, 2017, the Company entered into a First Amendment to Lease Agreement to amend the original lease agreement and to extend the term of the original lease for one year through January 31, 2019. On April 5, 2018, the Company entered into a Second Amendment to Lease Agreement to extend the lease term through January 31, 2020. On August 2, 2018, the Company entered into a Third Amendment to Lease Agreement to expand the size of the existing space for an additional base rent of $4,000 per month. On October 30, 2019, the Company entered into a Fourth Amendment to Lease Agreement to extend the lease term to approximately October 1, 2020, and to expand the size of the existing space for no additional base rent. All other terms and covenants from the original lease agreement remain unchanged. The Company's building lease is considered to be an operating lease. The lease agreement indicates the interest rate applicable to the lease is 12% , therefore the Company used a discount rate of 12% to calculate the value of its lease obligations. The Company re-measured the operating right-of-use asset and operating lease liability due to the Fourth Amendment to Lease Agreement. As of December 31, 2019 , the consolidated balance sheet includes a $0.6 million operating right-of-use asset within other long-term assets, and a $0.7 million operating lease liability in deferred revenue and other current liabilities. For the year ended December 31, 2019 , the Company recorded $0.4 million in operating lease cost, and the building lease has a remaining lease term of under one year from December 31, 2019 . As of December 31, 2019 , remaining lease payments on an undiscounted basis are $0.3 million for 2020. On August 22, 2019, the Company entered into a new lease agreement for office and laboratory space located in San Diego, California, for the Company's future corporate headquarters. The commencement date of this lease is expected to be September 21, 2020 and will expire October 1, 2030, unless terminated earlier. The base rent for the Company under this lease will be approximately $3.8 million for the first year of the lease, which amount will increase by 3% per year over the lease term. The Company has also received customary incentives from the landlord for tenant improvements and rent abatement periods, which effectively reduce the total lease payments owed for the lease. As of December 31, 2019 , the Company had not taken control of the space and the lease term had not commenced. Accordingly, no

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2019
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, 2019 and 2018 (unaudited, in thousands, except for per share data): Three Months Ended Year Ended 3/31/19 6/30/19 9/30/19 12/31/19 December 31, 2019 License and collaboration revenues $ 1,244 $ 577 $ 988 $ 526 $ 3,335 Loss from operations (42,758 ) (47,641 ) (57,059 ) (74,646 ) $ (222,104 ) Net loss (40,912 ) (45,695 ) (54,273 ) (72,376 ) (213,256 ) Basic and diluted net loss per share $ (1.17 ) $ (1.26 ) $ (1.38 ) $ (1.83 ) $ (5.69 ) Three Months Ended Year Ended 3/31/18 6/30/18 9/30/18 12/31/18 December 31, 2018 License and collaboration revenues $ 9,467 $ — $ — $ 3,459 $ 12,926 Loss from operations (15,346 ) (28,670 ) (28,939 ) (29,672 ) (102,627 ) Net loss (14,709 ) (27,869 ) (27,568 ) (28,272 ) (98,418 ) Basic and diluted net loss per share $ (0.51 ) $ (0.94 ) $ (0.85 ) $ (0.87 ) $ (3.19 )

Subsequent Event

Subsequent Event	12 Months Ended
	Dec. 31, 2019
Subsequent Events [Abstract]
Subsequent Event	Subsequent Event Sale of Common Stock 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 $324.1 million

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
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 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 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 stockholders' 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 a collaboration and license agreement 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 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 net loss.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets
Share-Based Compensation	Share-Based Compensation The Company measures and recognizes compensation expense for share-based payments based on estimated fair value, 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 is recognized using the graded accelerated vesting method.
Research and Development Expenses	Research and Development Expenses
General and Administrative Expenses	General and Administrative Expenses General and administrative expenses consist primarily of salaries and related benefits, including share-based compensation, related to our executive, finance, business development, legal, human resources and support functions. Other general and administrative expenses include professional fees for auditing, tax, consulting and patent-related services, rent and utilities and insurance.
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 common 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.
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, 2019
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, 2019 2018 2017 Common stock options 2,403,055 1,781,388 38,675 Common stock warrants 10,231,006 11,631,636 7,534,576 Total 12,634,061 13,413,024 7,573,251

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2019
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, 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 As of December 31, 2018 Maturity Amortized cost Gross unrealized gains Gross unrealized losses Estimated fair value Corporate debt securities 1 year or less $ 111,933 $ 26 $ (43 ) $ 111,916 Commercial paper 1 year or less 74,433 — (24 ) 74,409 U.S. Treasury bills 1 year or less 3,771 — — 3,771 $ 190,137 $ 26 $ (67 ) $ 190,096

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Schedule of Fair Value of Assets Measured on Recurring Basis	The following table summarizes the assets and liabilities measured at fair value on a recurring basis (in thousands): 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 December 31, 2018 Total Level 1 Level 2 Assets Cash and cash equivalents: Cash $ 3,731 $ 3,731 $ — Money market funds 28,963 28,963 — Total cash and cash equivalents 32,694 32,694 — Short-term investments: U.S. Treasury Bills 3,771 3,771 — Corporate debt securities 111,916 — 111,916 Commercial paper 74,409 — 74,409 Total short-term investments 190,096 3,771 186,325 Total $ 222,790 $ 36,465 $ 186,325

Other Current Assets and Othe_2

Other Current Assets and Other Long-Term Assets (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 Prepaid expenses $ 5,672 $ 1,261 Deposits and other receivables 2,119 1,841 Interest receivables 1,566 768 $ 9,357 $ 3,870

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): December 31, 2019 2018 Computer equipment $ 201 $ 201 Office and other equipment 329 260 Laboratory equipment 2,212 729 Leasehold improvements 63 63 Gross property and equipment 2,805 1,253 Less: Accumulated depreciation (1,029 ) (780 ) Property and equipment, net $ 1,776 $ 473

Accounts Payable and Accrued _2

Accounts Payable and Accrued Liabilities (Tables)	12 Months Ended
	Dec. 31, 2019
Payables and Accruals [Abstract]
Schedule of Accounts Payable and Accrued Liabilities	Accounts payable and accrued liabilities consisted of the following (in thousands): December 31, 2019 2018 Accounts payable $ 16,367 $ 8,531 Accrued clinical expense 21,290 10,154 Accrued development and other expense 2,510 1,243 Accrued compensation and benefits 7,915 5,847 $ 48,082 $ 25,775

BeiGene Agreement (Tables)

BeiGene Agreement (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 (in thousands): Opening balance, January 1, 2019 $ (481 ) Revenue from performance obligations satisfied during reporting period 309 Closing Balance, December 31, 2019 $ (172 )

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2019
Equity [Abstract]
Schedule of Shares Reserved for Future Issuance	The following shares were reserved for future issuance: December 31, 2019 Common stock options outstanding and available for future grant 7,923,098 Warrants to purchase common stock 9,692,879 Employee Stock Purchase Plan 142,872 17,758,849
Schedule of Warrants Issued and Outstanding	As of December 31, 2019 , 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 5,133,230 November 20, 2017 None $ 0.001 4,137,999 June 11, 2018 None $ 0.001 421,650 9,692,879

Share-Based Compensation (Table

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 : Number of options Weighted average exercise price Weighted-Average Remaining Contractual Term (years) Aggregate Intrinsic Value (millions) Balance outstanding as of December 31, 2018 3,683,786 $ 19.59 Granted 2,310,994 $ 76.22 Exercised (569,146 ) $ 14.89 Canceled/forfeited (137,657 ) $ 48.31 Balance outstanding as of December 31, 2019 5,287,977 $ 44.10 7.7 $ 448.2 Options exercisable at December 31, 2019 2,339,904 $ 21.11 6.1 $ 252.1
Schedule of Share-based Compensation Expense	Total share-based compensation expense by statement of operations and comprehensive loss classification is presented below (in thousands): Year ended December 31, 2019 2018 2017 Research and development expense $ 31,024 $ 7,232 $ 3,192 General and administrative expense 24,513 8,622 3,594 $ 55,537 $ 15,854 $ 6,786
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, 2019 2018 2017 Risk-free interest rate 2.2% 2.6% 2.1% Dividend yield —% —% —% Volatility factor 82.1% 94.3% 96.0% Expected term (in years) 5.6 6.0 6.0 Weighted average estimated fair value per share $52.03 $24.39 $4.17

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
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 2019 , 2018 and 2017 are as follows (in thousands): Year Ended December 31, 2019 2018 2017 Net loss before tax $ (213,256 ) $ (98,418 ) $ (70,430 ) Statutory combined U.S. federal and state tax rate 21.00 % 21.00 % 34.00 % Statutory federal and state taxes (44,784 ) (20,668 ) (23,946 ) Increase (decrease) in taxes recoverable resulting from: Effect of change in valuation allowance 52,719 25,959 (4,154 ) Non-deductible share-based compensation 1,810 884 695 Tax deductions for share-based compensation (6,917 ) (2,924 ) (80 ) Tax credits (8,621 ) (5,130 ) (2,563 ) Write off of Methylgene US Inc. net operating loss — — 307 Change in tax rate — — 303 Tax Cuts and Jobs Act — — 28,569 Uncertain tax positions 2,143 1,283 646 Return to provision and other true-ups (60 ) 375 394 Non-deductible officers' compensation 3,527 179 — Other differences 183 42 (171 ) Income tax benefit $ — $ — $ —
Schedule of Deferred Tax	The following table summarizes the significant components of our deferred tax assets (in thousands): December 31, 2019 2018 Deferred tax assets: Tangible and intangible depreciable assets $ 6,978 $ 8,123 Stock compensation 12,321 6,515 Provisions 934 1,020 Other, net 35 — Lease liability 137 — Net operating loss carry forwards 116,345 74,721 Capital loss carryforward 178 178 Canada scientific research and experimental development expenditures 5,471 5,467 U.S. research and development tax credits 17,080 10,613 Total gross deferred tax assets 159,479 106,637 Less valuation allowance (159,357 ) (106,637 ) Net deferred tax assets $ 122 $ — Deferred tax liabilities: Right-of-use Asset $ (122 ) $ — 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 — — 274 274 Does not expire 300,763 — — — $ 452,511 $ 81,161 $ 79,053 $ 78,432
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, 2019 2018 2017 2019 2018 2017 Unrecognized tax positions, beginning of year $ 2,617 $ 1,693 $ 1,095 $ 8,010 $ 7,556 $ 7,333 Gross increase — current period tax positions 1,651 924 588 638 454 227 Gross decrease — prior period tax positions — — — — — (3 ) Gross increase — prior period tax positions — — 11 — — — Expiration of statute of limitations — — (1 ) — — (1 ) Unrecognized tax positions, end of year $ 4,268 $ 2,617 $ 1,693 $ 8,648 $ 8,010 $ 7,556

Investment Tax Credits (Tables)

Investment Tax Credits (Tables)	12 Months Ended
	Dec. 31, 2019
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 1,018 $ 3,907

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 and 2018 (unaudited, in thousands, except for per share data): Three Months Ended Year Ended 3/31/19 6/30/19 9/30/19 12/31/19 December 31, 2019 License and collaboration revenues $ 1,244 $ 577 $ 988 $ 526 $ 3,335 Loss from operations (42,758 ) (47,641 ) (57,059 ) (74,646 ) $ (222,104 ) Net loss (40,912 ) (45,695 ) (54,273 ) (72,376 ) (213,256 ) Basic and diluted net loss per share $ (1.17 ) $ (1.26 ) $ (1.38 ) $ (1.83 ) $ (5.69 ) Three Months Ended Year Ended 3/31/18 6/30/18 9/30/18 12/31/18 December 31, 2018 License and collaboration revenues $ 9,467 $ — $ — $ 3,459 $ 12,926 Loss from operations (15,346 ) (28,670 ) (28,939 ) (29,672 ) (102,627 ) Net loss (14,709 ) (27,869 ) (27,568 ) (28,272 ) (98,418 ) Basic and diluted net loss per share $ (0.51 ) $ (0.94 ) $ (0.85 ) $ (0.87 ) $ (3.19 ) 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_4

Summary of Significant Accounting Policies - Estimated Useful Lives (Details)	12 Months Ended
	Dec. 31, 2019
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, 2019	Dec. 31, 2018	Dec. 31, 2017
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, 2019	Dec. 31, 2018	Dec. 31, 2017
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 (in shares)	12,634,061	13,413,024	7,573,251
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 (in shares)	2,403,055	1,781,388	38,675
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 (in shares)	10,231,006	11,631,636	7,534,576

Recently Issued and Recently _2

Recently Issued and Recently Adopted Accounting Pronouncements - Narrative (Details) - USD ($)	Dec. 31, 2019	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Right-of-use assets	$ 600,000
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease liability		$ 367,000
Right-of-use assets		$ 367,000

Investments - Summary (Details)

Investments - Summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 368,146	$ 190,137
Gross unrealized gains	375	26
Gross unrealized losses	(6)	(67)
Estimated fair value	$ 368,515	$ 190,096
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Maturity (or less)	2 years	1 year
Amortized cost	$ 160,065	$ 111,933
Gross unrealized gains	233	26
Gross unrealized losses	(1)	(43)
Estimated fair value	$ 160,297	$ 111,916
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Maturity (or less)	1 year	1 year
Amortized cost	$ 120,862	$ 74,433
Gross unrealized gains	74	0
Gross unrealized losses	0	(24)
Estimated fair value	$ 120,936	$ 74,409
U.S. Agency Bonds
Debt Securities, Available-for-sale [Line Items]
Maturity (or less)	2 years
Amortized cost	$ 50,745
Gross unrealized gains	41
Gross unrealized losses	(4)
Estimated fair value	$ 50,782
U.S. Treasury bills
Debt Securities, Available-for-sale [Line Items]
Maturity (or less)	2 years	1 year
Amortized cost	$ 36,474	$ 3,771
Gross unrealized gains	27	0
Gross unrealized losses	(1)	0
Estimated fair value	$ 36,500	$ 3,771

Fair Value Measurements - Summa

Fair Value Measurements - Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 46,535	$ 32,694
Short-term investments	368,515	190,096
Fair value of assets	415,050	222,790
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	46,535	32,694
Short-term investments	36,500	3,771
Fair value of assets	83,035	36,465
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Short-term investments	332,015	186,325
Fair value of assets	332,015	186,325
U.S. Treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	36,500	3,771
U.S. Treasury bills | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	36,500	3,771
U.S. Treasury bills | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	0	0
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	160,297	111,916
Corporate debt securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	0	0
Corporate debt securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	160,297	111,916
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	120,936	74,409
Commercial paper | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	0	0
Commercial paper | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	120,936	74,409
U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	50,782
U.S. Agency Bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	0
U.S. Agency Bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	50,782
Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	662	3,731
Cash | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	662	3,731
Cash | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	45,873	28,963
Money market funds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	45,873	28,963
Money market funds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 0	$ 0

Other Current Assets and Othe_3

Other Current Assets and Other Long-Term Assets - Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid expenses	$ 5,672	$ 1,261
Deposits and other receivables	2,119	1,841
Interest receivables	1,566	768
Other current assets	$ 9,357	$ 3,870

Other Current Assets and Othe_4

Other Current Assets and Other Long-Term Assets - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Other Assets, Noncurrent [Abstract]
Deposits paid for research and development	$ 5.1	$ 1.3
Operating right-of-use assets	0.6
Security deposit	$ 0.3

Property and Equipment, Net - S

Property and Equipment, Net - Summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property and equipment
Gross property and equipment	$ 2,805	$ 1,253
Less: Accumulated depreciation	(1,029)	(780)
Property and equipment, net	1,776	473
Depreciation expenses	249	175	$ 184
Computer equipment
Property and equipment
Gross property and equipment	201	201
Office and other equipment
Property and equipment
Gross property and equipment	329	260
Laboratory equipment
Property and equipment
Gross property and equipment	2,212	729
Leasehold improvements
Property and equipment
Gross property and equipment	$ 63	$ 63

Accounts Payable and Accrued _3

Accounts Payable and Accrued Liabilities - Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Accounts payable	$ 16,367	$ 8,531
Accrued clinical expense	21,290	10,154
Accrued development and other expense	2,510	1,243
Accrued compensation and benefits	7,915	5,847
Total accounts payable and accrued liabilities	$ 48,082	25,775
Noncurrent deferred revenue		$ 100

Accounts Payable and Accrued _4

Accounts Payable and Accrued Liabilities - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Long-term liabilities	$ 999	$ 732
Noncurrent deferred revenue		100
Other liabilities		$ 600

BeiGene Agreement - Narrative (

BeiGene Agreement - Narrative (Details) - Collaboration and License Agreement - USD ($)	Dec. 31, 2018	Jan. 07, 2018	Dec. 31, 2019	Dec. 31, 2018
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Up-front fee received		$ 10,000,000
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 obligations satisfied during reporting period			$ 309,000
Revenue from performance obligation expected to be earned		$ 123,000,000
Licenses of Intellectual Property
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from performance obligation earned			0	$ 9,500,000
Manufacturing Supply Services
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from performance obligations satisfied during reporting period			300,000	100,000
Revenue from performance obligation earned	$ 500,000		3,000,000	400,000
Increase (Decrease) in Contract with Customer, Liability			(3,300,000)	(500,000)
Cost-sharing receivable			400,000
Milestone Payments
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from performance obligation earned				$ 3,000,000
Revenue from performance obligation expected to be earned			5,000,000
Royalties
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue			$ 0

BeiGene Agreement - Activity in

BeiGene Agreement - Activity in Contract Liabilities (Details) - Collaboration and License Agreement $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Movement in Deferred Revenue [Roll Forward]
Opening balance at beginning of period	$ (481)
Revenue from performance obligations satisfied during reporting period	309
Closing balance at end of period	$ (172)

Stockholders' Equity - Stock Re

Stockholders' Equity - Stock Reserved for Future Issuance (Details)	Dec. 31, 2019shares
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	17,758,849
Warrants to purchase common stock
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	9,692,879
Stock Option Plan
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	7,923,098
Employee Stock Purchase Plan
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	142,872

Stockholders' Equity - Narrativ

Stockholders' Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended					12 Months Ended
	Jun. 30, 2019	Jan. 31, 2019	Jun. 30, 2018	Nov. 30, 2017	Jan. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Warrants - issued and outstanding
Stock issued in transaction (shares)	2,415,000	1,854,838	3,162,500
Sale price of common stock (USD per share)	$ 97	$ 62	$ 38.85
Proceeds from sale of common stock	$ 219,900	$ 107,900	$ 130,700
Exercise price of warrants (USD per share)			$ 0.001
Proceeds from disgorgement of stockholders' short-swing profits		$ 1,100				$ 1,050	$ 0	$ 0
Number of warrants exercised via cashless exercises (shares)						2,125,033	0	52,825
Net exercise of warrants (shares)						2,125,000		638,554
Exercise of warrants for cash (shares)								585,729
Proceeds from warrant exercises								$ 4,600
Warrant
Warrants - issued and outstanding
Stock issued in transaction (shares)			421,650	4,137,999	7,258,263
Sale price of common stock (USD per share)			$ 38.849	$ 12.999	$ 5.599
Maximum
Warrants - issued and outstanding
Proportion of common stock owned (as a percent)			9.99%
Minimum
Warrants - issued and outstanding
Proportion of common stock owned (as a percent)			4.99%

Stockholders' Equity - Warrants

Stockholders' Equity - Warrants Issued and Outstanding (Details) - $ / shares	Dec. 31, 2019	Jun. 30, 2018	Jun. 11, 2018	Nov. 20, 2017	Jan. 11, 2017
Class of Stock [Line Items]
Exercise price of warrants (USD per share)		$ 0.001
Number of warrants (shares)	9,692,879
Private Placement | January 11, 2017 Warrants
Class of Stock [Line Items]
Exercise price of warrants (USD per share)					$ 0.001
Number of warrants (shares)	5,133,230
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)	421,650

Share-Based Compensation - Narr

Share-Based Compensation - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants during period (shares)	2,310,994
Exercise price required percentage of fair market value	100.00%
Award vesting period	4 years
Intrinsic value of options exercised	$ 38,600,000	$ 18,800,000	$ 300,000
Cash received from exercise of stock options	8,500,000	9,700,000	400,000
Fair value of options vested	20,400,000	8,200,000	10,600,000
Capitalized costs	0	0	0
Tax benefit from compensation expense	0	$ 0	$ 0
Compensation cost not yet recognized	$ 77,400,000
Compensation cost not yet recognized, period for recognition	1 year 3 months 18 days
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)	2,200,000
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)	157,128
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	417,343
Grants during period (shares)	0

Share-Based Compensation - Stoc

Share-Based Compensation - Stock Option Activity (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Balance at beginning of year (shares) | shares	3,683,786
Granted (shares) | shares	2,310,994
Exercised (shares) | shares	(569,146)
Canceled (shares) | shares	(137,657)
Balance at end of year (shares) | shares	5,287,977
Options exercisable at end of year (shares) | shares	2,339,904
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	$ 19.59
Granted (USD per share) | $ / shares	76.22
Exercised (USD per share) | $ / shares	14.89
Canceled (USD per share) | $ / shares	48.31
Balance at end of year (USD per share) | $ / shares	44.10
Options exercisable at end of year (USD per share) | $ / shares	$ 21.11
Options outstanding, Weighted-Average Remaining Contractual Term (years)	7 years 8 months 12 days
Options exercisable, Weighted-Average Remaining Contractual Term (years)	6 years 1 month 6 days
Options outstanding, Aggregate Intrinsic Value | $	$ 448.2
Options exercisable, Aggregate Intrinsic Value | $	$ 252.1

Share-Based Compensation - Shar

Share-Based Compensation - Share-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 55,537	$ 15,854	$ 6,786
Research and development expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	31,024	7,232	3,192
General and administrative expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 24,513	$ 8,622	$ 3,594

Share-Based Compensation - Assu

Share-Based Compensation - Assumptions Used in Fair Value Estimates (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Payment Arrangement [Abstract]
Risk-free interest rate (as a percent)	2.20%	2.60%	2.10%
Dividend yield (as a percent)	0.00%	0.00%	0.00%
Volatility factor (as a percent)	82.10%	94.30%	96.00%
Expected term (in years)	5 years 7 months 6 days	6 years	6 years
Weighted average estimated fair value per share (USD per share)	$ 52.03	$ 24.39	$ 4.17

Employee Benefit Plan - Narrati

Employee Benefit Plan - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Proportion of employee contributions matched (as a percent)	4.00%
Maximum employer contribution per employee	$ 2,500
Matching contribution amount	$ 200,000	$ 100,000	$ 100,000

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details)	12 Months Ended
	Dec. 31, 2019USD ($)ongoing_tax_audit	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Income Tax [Line Items]
Income tax expense (benefit)	$ 0	$ 0	$ 0
Amount of increase in valuation allowance	(52,700,000)
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)	79,053,000
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)	78,432,000
US Federal
Income Tax [Line Items]
Net operating loss carry forwards (NOLs)	452,511,000
US Federal | Research Tax Credit Carryforward
Income Tax [Line Items]
Tax credit carryforward	17,200,000
US State
Income Tax [Line Items]
Net operating loss carry forwards (NOLs)	81,161,000
US State | Research Tax Credit Carryforward
Income Tax [Line Items]
Tax credit carryforward	$ 7,200,000

Income Taxes - Effective Income

Income Taxes - Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Net loss before tax	$ (213,256)	$ (98,418)	$ (70,430)
Statutory combined U.S. federal and state tax rate (as a percent)	21.00%	21.00%	34.00%
Statutory federal and state taxes	$ (44,784)	$ (20,668)	$ (23,946)
Effect of change in valuation allowance	52,719	25,959	(4,154)
Non-deductible share-based compensation	1,810	884	695
Tax deductions for share-based compensation	(6,917)	(2,924)	(80)
Tax credits	(8,621)	(5,130)	(2,563)
Write off of Methylgene US Inc. net operating loss	0	0	307
Change in tax rate	0	0	303
Tax Cuts and Jobs Act	0	0	28,569
Uncertain tax positions	2,143	1,283	646
Return to provision and other true-ups	(60)	375	394
Non-deductible officers' compensation	3,527	179	0
Other differences	183	42	(171)
Income tax benefit	$ 0	$ 0	$ 0

Income Taxes - Net Deferred Inc

Income Taxes - Net Deferred Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Tangible and intangible depreciable assets	$ 6,978	$ 8,123
Stock compensation	12,321	6,515
Provisions	934	1,020
Other, net	35	0
Lease liability	137	0
Net operating loss carry forwards	116,345	74,721
Capital loss carryforward	178	178
Canada scientific research and experimental development expenditures	5,471	5,467
U.S. research and development tax credits	17,080	10,613
Total gross deferred tax assets	159,479	106,637
Less valuation allowance	(159,357)	(106,637)
Net deferred tax assets	122	0
Deferred tax liabilities:
Right-of-use Asset	(122)	0
Net deferred income taxes	$ 0	$ 0

Income Taxes - Net Operating Lo

Income Taxes - Net Operating Loss Expirations (Details) $ in Thousands	Dec. 31, 2019USD ($)
US Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	$ 452,511
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 | Does not expire
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	300,763
US State
Operating Loss Carryforwards [Line Items]
Net operating loss carry forwards (NOLs)	81,161
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 | 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)	79,053
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)	274
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)	78,432
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)	274
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, 2019	Dec. 31, 2018	Dec. 31, 2017
Canadian Federal
Reconciliation of the beginning and ending gross amounts of unrecognized tax positions
Unrecognized tax positions, beginning of year	$ 2,617	$ 1,693	$ 1,095
Gross increase — current period tax positions	1,651	924	588
Gross decrease — prior period tax positions	0	0	0
Gross increase — prior period tax positions	0	0	11
Expiration of statute of limitations	0	0	(1)
Unrecognized tax positions, end of year	4,268	2,617	1,693
Canadian Provincial
Reconciliation of the beginning and ending gross amounts of unrecognized tax positions
Unrecognized tax positions, beginning of year	8,010	7,556	7,333
Gross increase — current period tax positions	638	454	227
Gross decrease — prior period tax positions	0	0	(3)
Gross increase — prior period tax positions	0	0	0
Expiration of statute of limitations	0	0	(1)
Unrecognized tax positions, end of year	$ 8,648	$ 8,010	$ 7,556

Investment Tax Credits - Summar

Investment Tax Credits - Summary (Details) - Canadian Federal $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
INVESTMENT TAX CREDITS
Non-refundable ITCs related to scientific research and development expenditures	$ 3,907
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	$ 1,018

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2015
Lessee, Lease, Description [Line Items]
Right-of-use assets	$ 600,000
Current Headquarters
Lessee, Lease, Description [Line Items]
Discount rate (as a percent)	12.00%
Right-of-use assets	$ 600,000
Operating lease liability	700,000
Operating lease cost	$ 400,000
Remaining lease term	1 year
Remaining lease payments due in the next twelve months	$ 300,000
Future Headquarters
Lessee, Lease, Description [Line Items]
Right-of-use assets	0
Operating lease liability	0
Building
Lessee, Lease, Description [Line Items]
Annual rent increase (as a percent)		3.00%
Building | Current Headquarters
Lessee, Lease, Description [Line Items]
Minimum monthly rental expense	24,100
Addition to minimum monthly rental expense	4,000
Building | Future Headquarters
Lessee, Lease, Description [Line Items]
Minimum monthly rental expense	$ 3,800,000
Annual rent increase (as a percent)	3.00%

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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
License and collaboration revenues	$ 526	$ 988	$ 577	$ 1,244	$ 3,459	$ 0	$ 0	$ 9,467	$ 3,335	$ 12,926	$ 0
Loss from operations	(74,646)	(57,059)	(47,641)	(42,758)	(29,672)	(28,939)	(28,670)	(15,346)	(222,104)	(102,627)	(71,535)
Net loss	$ (72,376)	$ (54,273)	$ (45,695)	$ (40,912)	$ (28,272)	$ (27,568)	$ (27,869)	$ (14,709)	$ (213,256)	$ (98,418)	$ (70,430)
Loss per share, basic and diluted (USD per share)	$ (1.83)	$ (1.38)	$ (1.26)	$ (1.17)	$ (0.87)	$ (0.85)	$ (0.94)	$ (0.51)	$ (5.69)	$ (3.19)	$ (2.78)

Subsequent Event - Narrative (D

Subsequent Event - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	[1]	Dec. 31, 2017	[2]	Jun. 30, 2019	Jan. 31, 2019	Jun. 30, 2018
Subsequent Event [Line Items]
Public offering stock price (USD per share)							$ 97	$ 62	$ 38.85
Proceeds from issuance of common stock		$ 327,830	$ 130,663		$ 153,530
Common Stock | Subsequent Event
Subsequent Event [Line Items]
Issuance of common stock, net of costs (shares)	3,538,462
Public offering stock price (USD per share)	$ 97.50
Proceeds from issuance of common stock	$ 324,100
[1]	Proceeds in 2018 include 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.
[2]	Proceeds in 2017 include warrants to purchase up to 4,137,999 shares of the Company's common stock at a public offering price of $12.999 per warrant, net of issuance costs, and warrants to purchase up to 7,258,263 shares of the Company's common stock at a public offering price of $5.599 per warrant, net of issuance costs.

Uncategorized Items - mrtx-2019

Label	Element	Value
Restricted Cash, Noncurrent	us-gaap_RestrictedCashNoncurrent	$ 0
Restricted Cash, Noncurrent	us-gaap_RestrictedCashNoncurrent	0
Restricted Cash, Noncurrent	us-gaap_RestrictedCashNoncurrent	321,000
Additional Paid-in Capital [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	446,000
Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ (446,000)