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Stockholders' Equity - Special Participation Stock - Additional Information (Detail) (USD $)
1 Months Ended 12 Months Ended
Jul. 31, 2013
Dec. 31, 2013
Dec. 31, 2014
Nov. 30, 2013
Schedule Of Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]        
Number of shares of Special Participation Stock authorized for issuance   5,000,000us-gaap_PreferredStockSharesAuthorized 5,000,000us-gaap_PreferredStockSharesAuthorized 5,000,000us-gaap_PreferredStockSharesAuthorized
Outstanding shares of Special Participation Stock   0us-gaap_PreferredStockSharesOutstanding 0us-gaap_PreferredStockSharesOutstanding  
Special Participation Shares [Member]        
Schedule Of Share Based Compensation Arrangements By Share Based Payment Award Options [Line Items]        
Number of shares of Special Participation Stock authorized for issuance     10,000us-gaap_PreferredStockSharesAuthorized
/ us-gaap_StatementClassOfStockAxis
= kpti_SpecialParticipationStockMember
 
Share price (in dollars per share)     0.0001us-gaap_SharePrice
/ us-gaap_StatementClassOfStockAxis
= kpti_SpecialParticipationStockMember
 
Number of shares of common stock Issued in settlement of outstanding shares of Special Participation Stock 12,121kpti_StockIssuedDuringPeriodSharesConversionOfSpecialParticipationStock
/ us-gaap_StatementClassOfStockAxis
= kpti_SpecialParticipationStockMember
(10,000)kpti_StockIssuedDuringPeriodSharesConversionOfSpecialParticipationStock
/ us-gaap_StatementClassOfStockAxis
= kpti_SpecialParticipationStockMember
   
Outstanding shares of Special Participation Stock   0us-gaap_PreferredStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= kpti_SpecialParticipationStockMember
0us-gaap_PreferredStockSharesOutstanding
/ us-gaap_StatementClassOfStockAxis
= kpti_SpecialParticipationStockMember