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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) (USD $)
12 Months Ended 0 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jun. 28, 2013
Nov. 30, 2013
Jul. 26, 2013
Jan. 02, 2013
Foreign currency translation              
Net foreign exchange losses recognized in other income $ (200,000)us-gaap_ForeignCurrencyTransactionGainLossBeforeTax $ 1,300,000us-gaap_ForeignCurrencyTransactionGainLossBeforeTax          
Reclassification of Warrants              
Warrant liability 0us-gaap_DerivativeLiabilitiesCurrent 33,407,000us-gaap_DerivativeLiabilitiesCurrent         16,200,000us-gaap_DerivativeLiabilitiesCurrent
Initial fair value of warrants recorded within stockholders equity that was reversed             11,200,000us-gaap_WarrantsAndRightsOutstanding
Change in fair value related to periods prior to January 1, 2013 recorded as an increase to accumulated deficit             5,000,000us-gaap_NewAccountingPronouncementOrChangeInAccountingPrincipleCumulativeEffectOfChangeOnEquityOrNetAssets1
Change in fair value of warrant liability included in net loss 4,517,000us-gaap_FairValueAdjustmentOfWarrants 19,799,000us-gaap_FairValueAdjustmentOfWarrants 0us-gaap_FairValueAdjustmentOfWarrants        
Reclassification of Share-based compensation liability              
Share-based compensation liability           1,100,000us-gaap_DeferredCompensationShareBasedArrangementsLiabilityCurrent  
Increase in additional paid-in capital due to reclassification of share-based compensation liability   300,000us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition          
Increase in stock compensation expense   1,400,000mrtx_IncreaseDecreaseInAllocatedShareBasedCompensationExpense          
Fair value of liability options         $ 2,200,000mrtx_DeferredCompensationLiabilityFairValueDisclosure    
MethylGene              
Basis of presentation              
Ratio of reverse split of the common shares       50us-gaap_StockholdersEquityNoteStockSplitConversionRatio1
/ dei_LegalEntityAxis
= mrtx_QuebecInc92229129Member
     
Exercisable ratio of outstanding options and warrants       50mrtx_StockholdersEquityOptionsAndWarrantsExercisableRatio
/ dei_LegalEntityAxis
= mrtx_QuebecInc92229129Member