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STOCKHOLDERS' EQUITY (Details 2)
In Thousands, except Share data, unless otherwise specified
0 Months Ended 12 Months Ended
Sep. 12, 2014
Dec. 31, 2014
Dec. 31, 2013
USD ($)
Dec. 31, 2012
USD ($)
Dec. 31, 2014
Common Stock
Dec. 31, 2013
Common Stock
USD ($)
Dec. 31, 2012
Common Stock
USD ($)
Dec. 31, 2014
Private Placement April 2011
April 4, 2011 Warrants
Apr. 04, 2011
Private Placement April 2011
April 4, 2011 Warrants
Dec. 31, 2014
Private Placement November 2012
November 21, 2012 Warrants
Nov. 21, 2012
Private Placement November 2012
November 21, 2012 Warrants
Warrants - issued and outstanding                      
Exercise price (in dollars per share)               6.74us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights
/ us-gaap_ClassOfWarrantOrRightAxis
= mrtx_ClassOfWarrantsIssuedApril42011Member
/ mrtx_SaleOfStockAxis
= mrtx_PrivatePlacementApril42011Member
7.46us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights
/ us-gaap_ClassOfWarrantOrRightAxis
= mrtx_ClassOfWarrantsIssuedApril42011Member
/ mrtx_SaleOfStockAxis
= mrtx_PrivatePlacementApril42011Member
7.86us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights
/ us-gaap_ClassOfWarrantOrRightAxis
= mrtx_ClassOfWarrantsIssuedNovember212012Member
/ mrtx_SaleOfStockAxis
= mrtx_PrivatePlacementNovember212012Member
8.70us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights
/ us-gaap_ClassOfWarrantOrRightAxis
= mrtx_ClassOfWarrantsIssuedNovember212012Member
/ mrtx_SaleOfStockAxis
= mrtx_PrivatePlacementNovember212012Member
Number of warrants (in shares)   2,465,713us-gaap_ClassOfWarrantOrRightOutstanding           1,397,921us-gaap_ClassOfWarrantOrRightOutstanding
/ us-gaap_ClassOfWarrantOrRightAxis
= mrtx_ClassOfWarrantsIssuedApril42011Member
/ mrtx_SaleOfStockAxis
= mrtx_PrivatePlacementApril42011Member
  1,067,792us-gaap_ClassOfWarrantOrRightOutstanding
/ us-gaap_ClassOfWarrantOrRightAxis
= mrtx_ClassOfWarrantsIssuedNovember212012Member
/ mrtx_SaleOfStockAxis
= mrtx_PrivatePlacementNovember212012Member
 
Number of warrants exercised (in shares)   73,964mrtx_ClassOfWarrantOrRightExercised                  
Net exercise of warrants (in shares)   43,526mrtx_StockIssuedDuringPeriodSharesWarrantsExercisedNoncash     43,526mrtx_StockIssuedDuringPeriodSharesWarrantsExercisedNoncash
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
108,321mrtx_StockIssuedDuringPeriodSharesWarrantsExercisedNoncash
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
5,653mrtx_StockIssuedDuringPeriodSharesWarrantsExercisedNoncash
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
       
Assumptions used for calculation of fair value of warrants issued                      
Weighted average risk-free interest rate (as a percent) 1.20%us-gaap_FairValueAssumptionsRiskFreeInterestRate                    
Volatility factor (as a percent) 100.80%us-gaap_FairValueAssumptionsExpectedVolatilityRate                    
Expected life 3 years 2 months 12 days                    
Fair value of warrants       $ 4,942us-gaap_StockAndWarrantsIssuedDuringPeriodValuePreferredStockAndWarrants              
Amount recorded as increase in common stock     $ 54,196us-gaap_StockIssuedDuringPeriodValueNewIssues $ 19,886us-gaap_StockIssuedDuringPeriodValueNewIssues   $ 3us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember
$ 4us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_CommonStockMember