Label	Element	Value
Restricted Cash	us-gaap_RestrictedCash	$ 620,000
Restricted Cash	us-gaap_RestrictedCash	320,000
Pfizer Discovery and Collaboration Agreement [Member] | Sales Milestone Payments [Member]
Research and Development Arrangement, Discovery and Collaboration Agreement, Compensation Expected to be Paid	mrtx_ResearchandDevelopmentArrangementDiscoveryandCollaborationAgreementCompensationExpectedtobePaid	337,000,000.0
Pfizer Discovery and Collaboration Agreement [Member] | Milestone Payments - Initiation of First Phase 2 Trial for MRTX849 [Member]
Research and Development Expense	us-gaap_ResearchAndDevelopmentExpense	3,000,000.0
Pfizer Discovery and Collaboration Agreement [Member] | Milestone Payments - Initiation of First Regulatory Toxicology Study for MRTX1133 [Member]
Research and Development Expense	us-gaap_ResearchAndDevelopmentExpense	300,000
Pfizer Discovery and Collaboration Agreement [Member] | Development Milestone Payments [Member]
Research and Development Arrangement, Discovery and Collaboration Agreement, Compensation Expected to be Paid	mrtx_ResearchandDevelopmentArrangementDiscoveryandCollaborationAgreementCompensationExpectedtobePaid	9,300,000
Research and Development Arrangement, Discovery and Collaboration Agreement, Milestone Payments Paid	mrtx_ResearchandDevelopmentArrangementDiscoveryandCollaborationAgreementMilestonePaymentsPaid	$ 4,500,000