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License and Asset Purchase Agreements - Additional Information - Antengene License Agreement (Detail) - Antengene Therapeutics Limited [Member] - USD ($)		3 Months Ended
	May 23, 2018	Mar. 31, 2019
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues		$ 0
Deferred Revenue, current		10,650,000
Deferred Revenue, non-current		$ 1,053,000
Government Research Grant Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
License agreement termination prior notice period		180 days
License agreement termination description		License Agreement may be terminated earlier by (i) either party for breach of the Antengene License Agreement by the other party or in the event of the insolvency or bankruptcy of the other party, (ii) Antengene on a product-by-product basis for certain safety reasons or on a product-by-product, country-by-country basis for any reason with 180 days' prior notice or (iii) the Company in the event Antengene challenges or assists with a challenge to certain of the Company's patent rights.
Government Research Grant Agreement [Member] \| Development Milestone [Member] \| Maximum [Member] \| Scenario, Plan [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments receivable	$ 105,000,000
Government Research Grant Agreement [Member] \| Sales Milestone Events [Member] \| Maximum [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments receivable	45,000,000
Commercial milestone payments receivable	45,000,000
Government Research Grant Agreement [Member] \| Development Goals [Member] \| Maximum [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments receivable	105,000,000
Selinexor [Member] \| Government Research Grant Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Performance obligations		$ 9,363,000
Eltanexor [Member] \| Government Research Grant Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Performance obligations		1,053,000
KPT-9274 [Member] \| Government Research Grant Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Performance obligations		1,053,000
Verdinexor [Member] \| Government Research Grant Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Performance obligations		$ 234,000
Up-front Payment Arrangement [Member] \| Government Research Grant Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Upfront payments	$ 11,703,000