License Agreements - Additional Information (Details) - USD ($)	1 Months Ended				12 Months Ended
	Dec. 31, 2017	Sep. 30, 2016	Apr. 30, 2016	May 31, 2015	Dec. 31, 2018	Dec. 31, 2017
License Agreement [Line Items]
Research and development					$ 47,948,000	$ 21,378,000
City Of Hope
License Agreement [Line Items]
Annual license fee			$ 25,000
City Of Hope | Collaboration Agreement
License Agreement [Line Items]
Payment for execution of agreements	$ 4,500,000
City Of Hope | Sponsored Research Agreement
License Agreement [Line Items]
Research and development					100,000	$ 100,000
Contractual obligation					$ 600,000
City Of Hope | Maximum
License Agreement [Line Items]
Development and commercialization milestone payment			$ 3,200,000
City Of Hope | Maximum | Sponsored Research Agreement
License Agreement [Line Items]
Collaborative arrangement research and development services				$ 1,100,000
California Institute Of Technology | Collaboration Agreement
License Agreement [Line Items]
Payment for execution of agreements	$ 100,000
California Institute Of Technology | Maximum | Co Exclusive License Agreement
License Agreement [Line Items]
Development and regulatory milestone payments		$ 7,200,000
California Institute Of Technology | Minimum | Co Exclusive License Agreement
License Agreement [Line Items]
Annual minimal royalty fee		$ 20,000