Contract with Customer, Milestone Payments Receivable

No definition available.

Name:	mrtx_ContractwithCustomerMilestonePaymentsReceivable
Namespace Prefix:	mrtx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Research and Development Arrangement, Contract to Perform for Others, Compensation Expected to be Earned

+ References

No definition available.

+ Details

Name:	mrtx_ResearchandDevelopmentArrangementContracttoPerformforOthersCompensationExpectedtobeEarned
Namespace Prefix:	mrtx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Research and Development Arrangement, Contract to Perform for Others, Period Required for Notice of Termination of Contract

+ References

No definition available.

+ Details

Name:	mrtx_ResearchandDevelopmentArrangementContracttoPerformforOthersPeriodRequiredforNoticeofTerminationofContract
Namespace Prefix:	mrtx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Research and Development Arrangement, Contract to Perform for Others, Termination of Contract, Period after First Commercial Sale of Product

+ References

No definition available.

+ Details

Name:	mrtx_ResearchandDevelopmentArrangementContracttoPerformforOthersTerminationofContractPeriodafterFirstCommercialSaleofProduct
Namespace Prefix:	mrtx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Research and Development Arrangement, Discovery and Collaboration Agreement, Period Required for Notice of Termination of Contract

+ References

No definition available.

+ Details

Name:	mrtx_ResearchandDevelopmentArrangementDiscoveryandCollaborationAgreementPeriodRequiredforNoticeofTerminationofContract
Namespace Prefix:	mrtx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Research and Development Arrangement, Discovery and Collaboration Agreement, Termination of Contract, Period after First Commercial Sale of Product

+ References

No definition available.

+ Details

Name:	mrtx_ResearchandDevelopmentArrangementDiscoveryandCollaborationAgreementTerminationofContractPeriodafterFirstCommercialSaleofProduct
Namespace Prefix:	mrtx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Research and Development Arrangement, Up-Front Fee Received

+ References

No definition available.

+ Details

Name:	mrtx_ResearchandDevelopmentArrangementUpFrontFeeReceived
Namespace Prefix:	mrtx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130549-203045

+ Details

Name:	us-gaap_ContractWithCustomerLiabilityRevenueRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase (decrease) in obligation to transfer good or service to customer for which consideration has been received or is receivable.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

+ Details

Name:	us-gaap_IncreaseDecreaseInContractWithCustomerLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of compensation earned (contract income) under a research and development arrangement accounted for as a contract to perform research and development for others.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 730
-SubTopic 20
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6420387&loc=d3e23221-108380

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 730
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6420387&loc=d3e23199-108380

+ Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersCompensationEarned
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 985
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 730
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6420194&loc=d3e21568-108373

+ Details

Name:	us-gaap_ResearchAndDevelopmentExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Ancillary revenue earned during the period from the consideration paid to the entity for the use of its rights and property by another party. Examples include licensing the use of copyrighted materials and leasing the extraction of natural resources.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7)
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

+ Details

Name:	us-gaap_RoyaltyIncomeNonoperating
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis=mrtx_BeiGeneCollaborationandLicenseAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=us-gaap_LicenseAgreementTermsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=mrtx_ManufacturingSupplyServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=mrtx_MilestonePaymentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=us-gaap_RoyaltyAgreementTermsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis=mrtx_PfizerDiscoveryandCollaborationAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	mrtx_ResearchandDevelopmentArrangementDiscoveryandCollaborationAgreementTypeAxisAxis=mrtx_MilestonePaymentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	mrtx_ResearchandDevelopmentArrangementDiscoveryandCollaborationAgreementTypeAxisAxis=mrtx_ResearchandDevelopmentServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Collaboration Agreements - Narrative (Details) - USD ($)		1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jan. 07, 2018	Oct. 31, 2014	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2018
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Expenses related to collaboration agreement			$ 65,083,000	$ 38,324,000	$ 136,791,000	$ 72,564,000
BeiGene Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Up-front fee received	$ 10,000,000.0
Revenue from performance obligation expected to be earned	$ 123,000,000.0
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					29,000
BeiGene Agreement \| Licenses of Intellectual Property
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from performance obligation earned			0	0	0	0	$ 9,500,000
BeiGene Agreement \| Manufacturing Supply Services
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from performance obligation earned						1,600,000	$ 500,000
Payments received in advance			0	(600,000)	(300,000)	(1,800,000)
Revenue from performance obligations satisfied during reporting period						200,000
BeiGene Agreement \| Milestone Payments
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from performance obligation expected to be earned			5,000,000.0		5,000,000.0
Milestone payments earned			0	0	0	0
BeiGene Agreement \| Royalties
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue			0	0	0	0
Pfizer Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Termination of contract, period after first commercial sale of product		10 years
Period required for notice of termination of contract		60 days
Expenses related to collaboration agreement			$ 0	$ 1,500,000	4,500,000	4,000,000.0
Pfizer Agreement \| Milestone Payments
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Expenses related to collaboration agreement					3,000,000.0	1,000,000.0
Pfizer Agreement \| Research and Development Services
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Expenses related to collaboration agreement					$ 1,500,000	$ 3,000,000.0