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INCOME TAXES (Details) (USD $)
12 Months Ended
Dec. 31, 2014
ongoing_tax_audit
Dec. 31, 2013
Dec. 31, 2012
Current income tax expense      
Federal $ 0us-gaap_CurrentFederalTaxExpenseBenefit $ 23,000us-gaap_CurrentFederalTaxExpenseBenefit $ (30,000)us-gaap_CurrentFederalTaxExpenseBenefit
State 0us-gaap_CurrentStateAndLocalTaxExpenseBenefit 0us-gaap_CurrentStateAndLocalTaxExpenseBenefit (9,000)us-gaap_CurrentStateAndLocalTaxExpenseBenefit
Canada 0us-gaap_CurrentForeignTaxExpenseBenefit 0us-gaap_CurrentForeignTaxExpenseBenefit 0us-gaap_CurrentForeignTaxExpenseBenefit
Total current tax expense (benefit) 0us-gaap_CurrentIncomeTaxExpenseBenefit 23,000us-gaap_CurrentIncomeTaxExpenseBenefit (39,000)us-gaap_CurrentIncomeTaxExpenseBenefit
Deferred tax asset      
Tangible and intangible depreciable assets 185,000mrtx_DeferredTaxAssetsTangibleAndIntangibleDepreciableAssets 874,000mrtx_DeferredTaxAssetsTangibleAndIntangibleDepreciableAssets  
Stock compensation 2,360,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost 0us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost  
Manufactured drug product inventory to be used in research 1,425,000us-gaap_DeferredTaxAssetsInventory 1,459,000us-gaap_DeferredTaxAssetsInventory  
Provisions 554,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsReserves 93,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsReserves  
Financing fees 261,000mrtx_DeferredTaxAssetsFinancingFees 445,000mrtx_DeferredTaxAssetsFinancingFees  
Net operating loss carry forwards 23,243,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards 13,099,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards  
Scientific research and experimental development expenditures 5,715,000mrtx_DeferredTaxAssetsScientificResearchAndExperimentalDevelopmentExpenditures 5,766,000mrtx_DeferredTaxAssetsScientificResearchAndExperimentalDevelopmentExpenditures  
Research and development tax credits 266,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch 0us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch  
Total gross deferred tax assets 34,009,000us-gaap_DeferredTaxAssetsGross 21,736,000us-gaap_DeferredTaxAssetsGross  
Less valuation allowance (34,009,000)us-gaap_DeferredTaxAssetsValuationAllowance (21,736,000)us-gaap_DeferredTaxAssetsValuationAllowance  
Net deferred tax assets and liabilities 0us-gaap_DeferredTaxAssetsNet 0us-gaap_DeferredTaxAssetsNet  
Additional disclosures      
Amount of increase in valuation allowance (12,300,000)us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount    
Share issue expenses not deducted 1,000,000mrtx_IncomeTaxExpenseBenefitFinancingsCostsnotDeducted 1,700,000mrtx_IncomeTaxExpenseBenefitFinancingsCostsnotDeducted 2,400,000mrtx_IncomeTaxExpenseBenefitFinancingsCostsnotDeducted
Reconciliation of the beginning and ending gross amounts of unrecognized tax positions      
Unrecognized tax benefits that would not impact effective tax rate 100,000mrtx_UnrecognizedTaxBenefitsthatWouldnotImpactEffectiveTaxRate    
Accrual for interest or penalties on tax matters 0us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued 0us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued  
Number of ongoing tax audits 0mrtx_NumberOfOngoingTaxAudits    
Canadian Federal      
Additional disclosures      
Scientific research and experimental development expenses deductible for income tax purposes 20,100,000mrtx_ResearchAndDevelopmentExpenseDeductibleForFederalAndProvincialTaxPurposes
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
20,700,000mrtx_ResearchAndDevelopmentExpenseDeductibleForFederalAndProvincialTaxPurposes
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
15,200,000mrtx_ResearchAndDevelopmentExpenseDeductibleForFederalAndProvincialTaxPurposes
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
Net operating loss carry forwards (NOLs) 71,642,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
   
Reconciliation of the beginning and ending gross amounts of unrecognized tax positions      
Unrecognized tax positions, beginning of year 35,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
43,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
42,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
Gross decrease — current period tax positions 0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
Gross increase — current period tax positions 35,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
0us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
1,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
Gross decrease — prior period tax positions (28,000)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
(13,000)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
Gross increase — prior period tax positions 0us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
5,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
0us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
Unrecognized tax positions, end of year 42,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
35,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
43,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
Canadian Federal | 2030      
Additional disclosures      
Net operating loss carry forwards (NOLs) 5,907,000us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2030Member
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
   
Canadian Federal | 2031      
Additional disclosures      
Net operating loss carry forwards (NOLs) 7,059,000us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2031Member
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
   
Canadian Federal | 2032      
Additional disclosures      
Net operating loss carry forwards (NOLs) 13,312,000us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2032Member
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
   
Canadian Federal | 2033      
Additional disclosures      
Net operating loss carry forwards (NOLs) 18,623,000us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2033Member
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
   
Canadian Federal | 2034      
Additional disclosures      
Net operating loss carry forwards (NOLs) 26,741,000us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2034Member
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianFederalMember
   
Canadian Provincial      
Additional disclosures      
Scientific research and experimental development expenses deductible for income tax purposes 22,700,000mrtx_ResearchAndDevelopmentExpenseDeductibleForFederalAndProvincialTaxPurposes
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
22,400,000mrtx_ResearchAndDevelopmentExpenseDeductibleForFederalAndProvincialTaxPurposes
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
16,800,000mrtx_ResearchAndDevelopmentExpenseDeductibleForFederalAndProvincialTaxPurposes
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
Net operating loss carry forwards (NOLs) 71,018,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
   
Reconciliation of the beginning and ending gross amounts of unrecognized tax positions      
Unrecognized tax positions, beginning of year 6,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
2,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
1,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
Gross decrease — current period tax positions 0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
Gross increase — current period tax positions 12,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
0us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
1,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
Gross decrease — prior period tax positions 0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
Gross increase — prior period tax positions 0us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
4,000us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
0us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
Unrecognized tax positions, end of year 18,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
6,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
2,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
Canadian Provincial | 2030      
Additional disclosures      
Net operating loss carry forwards (NOLs) 5,985,000us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2030Member
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
   
Canadian Provincial | 2031      
Additional disclosures      
Net operating loss carry forwards (NOLs) 7,066,000us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2031Member
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
   
Canadian Provincial | 2032      
Additional disclosures      
Net operating loss carry forwards (NOLs) 12,433,000us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2032Member
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
   
Canadian Provincial | 2033      
Additional disclosures      
Net operating loss carry forwards (NOLs) 19,385,000us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2033Member
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
   
Canadian Provincial | 2034      
Additional disclosures      
Net operating loss carry forwards (NOLs) 26,149,000us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2034Member
/ us-gaap_IncomeTaxAuthorityAxis
= mrtx_CanadianProvincialMember
   
US Federal      
Additional disclosures      
Net operating loss carry forwards (NOLs) 10,273,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember
   
US Federal | 2030      
Additional disclosures      
Net operating loss carry forwards (NOLs) 0us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2030Member
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember
   
US Federal | 2031      
Additional disclosures      
Net operating loss carry forwards (NOLs) 0us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2031Member
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember
   
US Federal | 2032      
Additional disclosures      
Net operating loss carry forwards (NOLs) 0us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2032Member
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember
   
US Federal | 2033      
Additional disclosures      
Net operating loss carry forwards (NOLs) 3,261,000us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2033Member
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember
   
US Federal | 2034      
Additional disclosures      
Net operating loss carry forwards (NOLs) 7,012,000us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2034Member
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember
   
US State      
Additional disclosures      
Net operating loss carry forwards (NOLs) 9,471,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
   
US State | 2030      
Additional disclosures      
Net operating loss carry forwards (NOLs) 0us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2030Member
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
   
US State | 2031      
Additional disclosures      
Net operating loss carry forwards (NOLs) 0us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2031Member
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
   
US State | 2032      
Additional disclosures      
Net operating loss carry forwards (NOLs) 0us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2032Member
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
   
US State | 2033      
Additional disclosures      
Net operating loss carry forwards (NOLs) 2,286,000us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2033Member
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
   
US State | 2034      
Additional disclosures      
Net operating loss carry forwards (NOLs) 7,185,000us-gaap_OperatingLossCarryforwards
/ mrtx_ExpirationPeriodAxis
= mrtx_ExpirationPeriod2034Member
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
   
Research Tax Credit Carryforward [Member] | US Federal      
Additional disclosures      
Tax Credit Carryforward, Amount   200,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember
/ us-gaap_TaxCreditCarryforwardAxis
= us-gaap_ResearchMember
 
Research Tax Credit Carryforward [Member] | US State      
Additional disclosures      
Tax Credit Carryforward, Amount   $ 100,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
/ us-gaap_TaxCreditCarryforwardAxis
= us-gaap_ResearchMember