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INVESTMENTS (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 22,708mrtx_AvailableforsaleandTradingSecuritiesAmortizedCost $ 47,851mrtx_AvailableforsaleandTradingSecuritiesAmortizedCost
Gross unrealized gains 3mrtx_AvailableforsaleandTradingSecuritiesAccumulatedGrossUnrealizedGainbeforeTax 12mrtx_AvailableforsaleandTradingSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
Gross unrealized losses (1)mrtx_AvailableforsaleandTradingSecuritiesAccumulatedGrossUnrealizedLossbeforeTax (28)mrtx_AvailableforsaleandTradingSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
Estimated fair value 22,710mrtx_AvailableforsaleandTradingSecuritiesFairValue 47,835mrtx_AvailableforsaleandTradingSecuritiesFairValue
U.S. Treasury securities and debt securities of U.S. Government agencies    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale, maximum maturity period   2 years [1]
Available-for-Sale, Amortized cost   3,001us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		 [1]
Available-for-Sale, Gross unrealized gains   0mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		 [1]
Available-for-Sale, Gross unrealized losses   (3)mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		 [1]
Available-for-Sale, Estimated fair value   2,998us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		 [1]
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale, maximum maturity period 1 year [1] 2 years [1]
Available-for-Sale, Amortized cost 21,208us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 [1] 31,319us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 [1]
Available-for-Sale, Gross unrealized gains 3mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 [1] 0mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 [1]
Available-for-Sale, Gross unrealized losses (1)mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 [1] (22)mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 [1]
Available-for-Sale, Estimated fair value 21,210us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 [1] 31,297us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 [1]
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale, maximum maturity period 1 year [1] 1 year [1]
Available-for-Sale, Amortized cost 1,500us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperMember		 [1] 8,485us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperMember		 [1]
Available-for-Sale, Gross unrealized gains 0mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperMember		 [1] 12mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperMember		 [1]
Available-for-Sale, Gross unrealized losses 0mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperMember		 [1] 0mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperMember		 [1]
Available-for-Sale, Estimated fair value 1,500us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperMember		 [1] 8,497us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperMember		 [1]
Guaranteed investment certificates    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Trading, maximum maturity period   1 year [2]
Trading, Amortized cost   5,046us-gaap_TradingSecuritiesCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mrtx_GuaranteedInvestmentCertificatesMember		 [2]
Trading, Gross unrealized gains   0mrtx_TradingSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mrtx_GuaranteedInvestmentCertificatesMember		 [2]
Trading, Gross unrealized losses   (3)mrtx_TradingSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mrtx_GuaranteedInvestmentCertificatesMember		 [2]
Trading, Estimated fair value   $ 5,043us-gaap_TradingSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= mrtx_GuaranteedInvestmentCertificatesMember		 [2]
[1] Investments are designated as available-for-sale investments.
[2] Investments are designated as trading investments.