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Subsequent Events - Additional Information (Detail) (USD $)
In Thousands, except Share data, unless otherwise specified
12 Months Ended 0 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jan. 31, 2015
Subsequent Event [Line Items]        
Underwritten offering price per share $ 0.0001us-gaap_CommonStockParOrStatedValuePerShare $ 0.0001us-gaap_CommonStockParOrStatedValuePerShare    
Net proceeds after deducting underwriting discounts, commissions and expenses payable $ 112,837us-gaap_ProceedsFromIssuanceOfCommonStock $ 113,155us-gaap_ProceedsFromIssuanceOfCommonStock $ 9us-gaap_ProceedsFromIssuanceOfCommonStock  
Subsequent Event [Member] | Underwriting Offering [Member]        
Subsequent Event [Line Items]        
Common stock issued and sold pursuant to underwriters option to purchase shares       2,950,000us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
/ us-gaap_SubsidiarySaleOfStockAxis
= kpti_UnderwritingOfferingMember
Underwritten offering, par value price per share       $ 0.0001us-gaap_SaleOfStockPricePerShare
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
/ us-gaap_SubsidiarySaleOfStockAxis
= kpti_UnderwritingOfferingMember
Underwritten offering price per share       $ 33.00us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
/ us-gaap_SubsidiarySaleOfStockAxis
= kpti_UnderwritingOfferingMember
Net proceeds after deducting underwriting discounts, commissions and expenses payable       $ 90,868us-gaap_ProceedsFromIssuanceOfCommonStock
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
/ us-gaap_SubsidiarySaleOfStockAxis
= kpti_UnderwritingOfferingMember