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Stock-based Compensation - Summary of Stock Option Activity for Employees and Nonemployees (Detail) (Employee and Nonemployee Stock Option [Member], USD $)
In Thousands, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Employee and Nonemployee Stock Option [Member]
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock option activity, shares outstanding, at beginning of period 2,410,522us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
 
Stock option activity, shares granted 900,200us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
 
Stock option activity, shares exercised (106,528)us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
 
Stock option activity, shares forfeited (191,271)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
 
Stock option activity, shares outstanding, at end of period 3,012,923us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
2,410,522us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
Stock option activity, options vested or expected to vest at end of period 2,833,435us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
 
Stock option activity, options exercisable at end of period 896,017us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
 
Weighted average exercise price, options outstanding at beginning of period $ 7.85us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
 
Weighted average exercise price, options granted $ 39.18us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
 
Weighted average exercise price, options exercised $ 1.35us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
 
Weighted average exercise price, options forfeited $ 20.27us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
 
Weighted average exercise price, options outstanding at end of period $ 16.65us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
$ 7.85us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
Weighted average exercise price, options vested or expected to vest at end of period $ 16.48us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
 
Weighted average exercise price, options exercisable at end of period $ 5.53us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
 
Weighted average remaining contractual life outstanding 8 years 8 months 12 days 9 years 3 months 18 days
Weighted average exercise price vested or expected to vest 8 years 7 months 6 days  
Weighted average exercise price exercisable 7 years 9 months 18 days  
Aggregate intrinsic value outstanding $ 64,659us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
$ 36,717us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
Aggregate intrinsic value vested or expected to vest 61,271us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember
 
Aggregate intrinsic value exercisable $ 28,591us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1
/ us-gaap_AwardTypeAxis
= kpti_EmployeeAndNonEmployeeStockOptionMember