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Investments (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale, Amortized cost $ 53,709us-gaap_AvailableForSaleSecuritiesAmortizedCost $ 22,708us-gaap_AvailableForSaleSecuritiesAmortizedCost
Available-for-sale, Gross unrealized gain 16mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax 3mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
Available-for-sale, Gross unrealized losses (9)mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax (1)mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
Available-for-sale, Estimated fair value 53,716us-gaap_AvailableForSaleSecurities 22,710us-gaap_AvailableForSaleSecurities
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, maximum maturity period 2 years 1 year
Available-for-sale, Amortized cost 45,224us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 21,208us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Available-for-sale, Gross unrealized gain 2mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 3mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Available-for-sale, Gross unrealized losses (9)mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 (1)mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Available-for-sale, Estimated fair value 45,217us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 21,210us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, maximum maturity period 1 year 1 year
Available-for-sale, Amortized cost 8,485us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember		 1,500us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember
Available-for-sale, Gross unrealized gain 14mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember		 0mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedGainbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember
Available-for-sale, Gross unrealized losses 0mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember		 0mrtx_AvailableforsaleSecuritiesAccumulatedGrossUnrealizedLossbeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember
Available-for-sale, Estimated fair value $ 8,499us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember		 $ 1,500us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember