v3.26.1
Long-Term Obligations - Additional Information (Detail)
 1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
May 08, 2027
Oct. 11, 2026
USD ($)
Oct. 10, 2026
USD ($)
Sep. 10, 2026
USD ($)
Apr. 01, 2026
Oct. 07, 2025
USD ($)
$ / shares
shares
May 13, 2024
USD ($)
shares
Oct. 31, 2025
USD ($)
$ / shares
shares
May 31, 2024
USD ($)
shares
Sep. 30, 2019
USD ($)
Jun. 30, 2026
USD ($)
$ / shares
Jun. 30, 2026
USD ($)
Days
$ / shares
shares
Oct. 10, 2026
USD ($)
Dec. 31, 2025
USD ($)
Oct. 30, 2025
May 08, 2024
USD ($)
Oct. 31, 2018
USD ($)
Subsequent Event [Line Items]                                  
Cash, cash equivalents and investments                     $ 65,100,000 $ 65,100,000          
Prepayment premium                       3.00%          
Prepayment premium thereafter                       0.00%          
Debt instrument exchange amount                 $ 148,000,000                
Exercise price | $ / shares               $ 6.64     $ 0.0001 $ 0.0001          
Line of Credit [Member]                                  
Subsequent Event [Line Items]                                  
Quarterly cash payments         6.25%                        
Secured Overnight Financing Rate [Member] | Line of Credit [Member]                                  
Subsequent Event [Line Items]                                  
Rate margin               10.25%       9.25%          
Base Rate [Member] | Line of Credit [Member]                                  
Subsequent Event [Line Items]                                  
Rate margin               3.00%       3.00%          
May 2024 Warrants [Member] | Maximum [Member]                                  
Subsequent Event [Line Items]                                  
Warrants purchased | shares             3,051,750   3,051,750                
Scenario, Forecast [Member]                                  
Subsequent Event [Line Items]                                  
Prepayment premium 5.00%                                
Minimum liquidity covenant amount   $ 25,000,000                     $ 25,000,000        
Scenario, Forecast [Member] | Line of Credit [Member]                                  
Subsequent Event [Line Items]                                  
First principal payment       $ 15,800,000                          
Location, Statement of Financial Position, Balance [Axis]: kpti:DeferredRoyaltyObligationNoncurrent | Level 3 [Member]                                  
Subsequent Event [Line Items]                                  
Deferred royalty obligation at fair value                     $ 72,300,000 $ 72,300,000   $ 72,300,000      
HealthCare Royalty Partners IV LP [Member]                                  
Subsequent Event [Line Items]                                  
Maximum remaining amount                 $ 128,300,000                
Revenue Interest Financing Agreement [Member]                                  
Subsequent Event [Line Items]                                  
Debt issuance costs                     $ 2,800,000 $ 2,800,000          
First investment amount                   $ 135,000,000              
Aggregate Royalties                   $ 263,300,000              
Revenue Interest Financing Agreement [Member] | HealthCare Royalty Partners IV LP [Member]                                  
Subsequent Event [Line Items]                                  
Repayment terms                       term may be shortened or extended depending on actual worldwide net product sales and upfront payments, milestones, and royalties. The repayment period expires on the earlier of (i) the date on which HCRx has received cash payments in the aggregate totaling $263.3 million or (ii) the legal maturity date of September 26, 2035. If HCRx has not received total payments in the aggregate equal to $263.3 million by September 26, 2035, we will be required to pay an amount equal to $135.0 million plus a specific annual rate of return less aggregate payments previously paid to HCRx. In the event of a change of control, an event of default, including, among others, our failure to pay any amounts due to HCRx, insolvency, our failure to pay indebtedness when due, the revocation of regulatory approval of XPOVIO in the U.S. or our breach of any covenant contained in the Amended Revenue Interest Agreement and our failure to cure the breach within the prescribed time frame, we are obligated to pay HCRx an amount equal to $263.3 million less aggregate payments previously paid to HCRx          
Revenue Interest Financing Agreement [Member] | KKR & Co. Inc. [Member]                                  
Subsequent Event [Line Items]                                  
Repayment terms                       term may be shortened or extended depending on actual worldwide net product sales and upfront payments, milestones, and royalties. The repayment period expires on the earlier of (i) the date on which HCRx has received cash payments in the aggregate totaling $263.3 million or (ii) the legal maturity date of September 26, 2035. If HCRx has not received total payments in the aggregate equal to $263.3 million by September 26, 2035, we will be required to pay an amount equal to $135.0 million plus a specific annual rate of return less aggregate payments previously paid to HCRx. In the event of a change of control, an event of default, including, among others, our failure to pay any amounts due to HCRx, insolvency, our failure to pay indebtedness when due, the revocation of regulatory approval of XPOVIO in the U.S. or our breach of any covenant contained in the Amended Revenue Interest Agreement and our failure to cure the breach within the prescribed time frame, we are obligated to pay HCRx an amount equal to $263.3 million less aggregate payments previously paid to HCRx          
Amended Revenue Interest Agreement [Member]                                  
Subsequent Event [Line Items]                                  
Aggregate principal amount                 135,000,000                
Revenue Interest Agreement and Amended Revenue Interest Agreement [Member]                                  
Subsequent Event [Line Items]                                  
Debt instrument interest rate                     16.00% 16.00%          
Revenue Interest Agreement and Amended Revenue Interest Agreement [Member] | HealthCare Royalty Partners IV LP [Member]                                  
Subsequent Event [Line Items]                                  
Cumulative payments                 49,500,000   $ 147,100,000 $ 147,100,000          
Term Loan [Member]                                  
Subsequent Event [Line Items]                                  
Aggregate principal amount                 15,000,000                
6% Convertible Senior Notes Due 2029 [Member]                                  
Subsequent Event [Line Items]                                  
Aggregate principal amount                 111,000,000                
Notes, interest rate                       6.00%          
6% Convertible Senior Notes Due 2029 [Member] | HealthCare Royalty Partners IV LP [Member]                                  
Subsequent Event [Line Items]                                  
Aggregate principal amount                 $ 5,000,000                
Credit Agreement and Term Loan [Member] | Line of Credit [Member]                                  
Subsequent Event [Line Items]                                  
Line of credit facility                               $ 100,000,000  
2029 Notes [Member]                                  
Subsequent Event [Line Items]                                  
Aggregate principal amount           $ 101,000,000 $ 5,000,000       $ 0 $ 0          
Notes, interest rate                       6.00%          
2029 Notes [Member] | Common Stock [Member]                                  
Subsequent Event [Line Items]                                  
Notes converted in to common stock, amount | shares           2,024,344                      
Pre-funded warrants to common stock | shares           552,164                      
2029 Notes [Member] | HealthCare Royalty Partners IV LP [Member]                                  
Subsequent Event [Line Items]                                  
Aggregate principal amount             5,000,000                    
2029 Notes [Member] | Convertible Note Offering [Member]                                  
Subsequent Event [Line Items]                                  
Notes, maturity date                       May 13, 2029          
Notes converted in to common stock, amount | shares                       29.6296          
Notes converted in to common stock, shares           $ 15,000,000 1,000                    
Proceeds from convertible debt                       $ 111,000,000          
Notes, conversion price per share | $ / shares                     $ 33.75 $ 33.75          
2025 Notes [Member]                                  
Subsequent Event [Line Items]                                  
Aggregate principal amount                                 $ 172,500,000
Debt instrument exchange amount               $ 24,300,000                  
Repayment of principal amount               $ 300,000                  
2025 Notes [Member] | Common Stock [Member]                                  
Subsequent Event [Line Items]                                  
Warrants purchased | shares               2,502,151,000                  
Notes converted in to common stock, amount | shares               2,435,146                  
Pre-funded warrants to common stock | shares               1,404,087                  
Exercise price | $ / shares               $ 6.64                  
Expiration date                             Oct. 10, 2030    
2025 Notes [Member] | Convertible Note Offering [Member]                                  
Subsequent Event [Line Items]                                  
Notes converted in to common stock, shares             $ 148,000,000                    
Amended Credit Agreement [Member]                                  
Subsequent Event [Line Items]                                  
Additional borrowings               $ 12,500,000                  
Amended Term Loan [Member]                                  
Subsequent Event [Line Items]                                  
Aggregate principal amount                     $ 128,981,000 $ 128,981,000   120,398,000      
Unamortized debt issuance costs amortized                     $ 5,200,000 $ 5,200,000          
Interest, payment terms               Interest incurred on borrowings under the Amended Term Loan from July 1, 2025 to June 30, 2026 was payable in-kind. Interest incurred on borrowings under the Amended Term Loan after June 30, 2026 is payable in cash quarterly in arrears on March 31, June 30, September 30, and December 31 of each year                  
Debt instrument interest rate                     16.00% 16.00%          
New 2029 Notes [Member]                                  
Subsequent Event [Line Items]                                  
Aggregate principal amount           $ 103,500,000         $ 107,974,000 $ 107,974,000   105,598,000      
Notes, maturity date           May 13, 2029                      
Notes converted in to common stock, amount | shares                       85.1374          
Notes converted in to common stock, shares                       $ 1,000          
Notes conversion price, percentage                       130.00%          
Estimated fair value of convertible notes                     $ 100,700,000 $ 100,700,000   92,700,000      
Amortization of debt issuance costs                       $ 33,300,000          
Debt instrument interest rate                     22.00% 22.00%          
New 2029 Notes [Member] | Convertible Note Offering [Member]                                  
Subsequent Event [Line Items]                                  
Notes converted in to common stock, amount | shares                       44.4444          
Notes converted in to common stock, shares                       $ 1,000          
Notes, conversion price per share | $ / shares                     $ 22.5 $ 22.5          
2028 Notes [Member]                                  
Subsequent Event [Line Items]                                  
Aggregate principal amount           $ 15,000,000         $ 15,648,000 $ 15,648,000   15,304,000      
Debt issuance costs                     5,600,000 5,600,000          
Interest expense on debt                     700,000 $ 1,300,000          
Notes converted in to common stock, amount | shares           150.6024           173.3102          
Notes converted in to common stock, shares           $ 1,000           $ 1,000          
Proceeds from convertible debt           $ 15,000,000                      
Notes, conversion price per share | $ / shares           $ 6.64                      
Notes conversion price, percentage                       165.00%          
Estimated fair value of convertible notes                     27,500,000 $ 27,500,000   $ 22,800,000      
Amortization of debt issuance costs                     300,000 600,000          
Contractual interest expense                     $ 400,000 $ 700,000          
Debt instrument interest rate                     25.00% 25.00%          
2029 and 2028 Notes [Member]                                  
Subsequent Event [Line Items]                                  
Notes, interest rate                       9.00%          
Interest, payment terms                       Interest incurred after March 31, 2026 will be payable in cash quarterly in arrears on March 31, June 30, September 30, and December 31 of each year.          
Notes instrument, trading days | Days                       20          
Debt instrument convertible threshold consecutive trading days | Days                       30          
Percentage of redeem notes at premium with proceeds                       101.00%          
2029 and 2028 Notes [Member] | Maximum [Member]                                  
Subsequent Event [Line Items]                                  
Percentage common stock outstanding                     19.99% 19.99%          
2029 and 2028 Notes [Member] | Minimum [Member]                                  
Subsequent Event [Line Items]                                  
Percentage common stock outstanding                     4.99% 4.99%          
2029 and 2028 Notes [Member] | Scenario, Forecast [Member]                                  
Subsequent Event [Line Items]                                  
Minimum liquidity covenant amount   $ 25,000,000                              
2029 and 2028 Notes [Member] | Scenario, Forecast [Member] | Maximum [Member]                                  
Subsequent Event [Line Items]                                  
Minimum liquidity covenant amount     $ 10,000,000                            
Minimum liquidity covenant percentage     50.00%                   50.00%