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STOCK-BASED COMPENSATION (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
May 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Options available for issue 400,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant      
Intrinsic value of options exercised $ 600,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue $ 200,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue $ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue  
Cash received from exercise of stock options 900,000us-gaap_EmployeeServiceShareBasedCompensationCashReceivedFromExerciseOfStockOptions 500,000us-gaap_EmployeeServiceShareBasedCompensationCashReceivedFromExerciseOfStockOptions    
Exercise of options (in shares) 76,224us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised      
Fair value of options vested 3,000,000us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1 2,000,000us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1 2,400,000us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1  
Capitalized costs 0us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount 0us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount 0us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount  
Tax benefit from compensation expense 0us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense   0us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense  
Compensation cost not yet recognized 7,100,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized      
Compensation cost not yet recognized, period for recognition 1 year 5 months 24 days      
Exercise price required percentage of fair market value 100.00%mrtx_SharebasedCompensationArrangementbySharebasedPaymentAwardExercisePriceRequiredPercentageofFairMarketValue      
Award vesting period 4 years      
Stock Option Plan | Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock option contractual terms 5 years      
Stock Option Plan | Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock option contractual terms 7 years      
2013 Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Employee Stock Purchase Plan, Shares Authorized for Purchase $ 25,000mrtx_EmployeeStockPurchasePlanSharesAuthorizedforPurchase
/ us-gaap_PlanNameAxis
= mrtx_A2013EquityIncentivePlanMember
     
Employee Stock Purchase Plan, Purchase Discount, Percent 85.00%mrtx_EmployeeStockPurchasePlanPurchaseDiscountPercent
/ us-gaap_PlanNameAxis
= mrtx_A2013EquityIncentivePlanMember
     
Employee Stock Purchase Plan, Shares Reserved for Future Issuance       300,000mrtx_EmployeeStockPurchasePlanSharesReservedforFutureIssuance
/ us-gaap_PlanNameAxis
= mrtx_A2013EquityIncentivePlanMember
Stock Issued During Period, Shares, Employee Stock Purchase Plans 0us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans
/ us-gaap_PlanNameAxis
= mrtx_A2013EquityIncentivePlanMember
     
2013 Plan | Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock option contractual terms 7 years      
2013 Plan | Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock option contractual terms 10 years