Overview and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	9 Months Ended	65 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds from common stock offering	$ 54,120	$ 630,800
Proceeds from issuance of convertible preferred stock		85,000
Collaborative Arrangement [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds from common stock offering		150,900
Initial Public Offering and Concurrent Private Placements [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds from common stock offering		170,500
Follow-on public Offering [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds from common stock offering		141,000
At-the-market Offering [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds from common stock offering		$ 83,400