Document and Entity Information - shares
6 Months Ended
Jun. 30, 2019
Jul. 31, 2019
Document and Entity Information [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
Current Fiscal Year End Date --12-31  
Entity Registrant Name Karyopharm Therapeutics Inc.  
Trading Symbol KPTI  
Entity Central Index Key 0001503802  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Common Stock, Shares Outstanding   61,920,517
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity File Number 001-36167  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 26-3931704  
Entity Address, Address Line One 85 Wells Avenue, 2nd Floor  
Entity Address, City or Town Newton  
Entity Address, Postal Zip Code 02459  
City Area Code 617  
Local Phone Number 658-0600  
Document Transition Report false  
Document Quarterly Report true  
Entity Shell Company false  
Entity Address, State or Province MA  
Title of 12(b) Security Common Stock  
Security Exchange Name NASDAQ