Organization and Operations - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Company incorporation date									Oct. 31, 2013
Accumulated deficit	$ (224,711)				$ (291,569)				$ (224,711)	$ (291,569)
Cash and cash equivalents	943,771				456,649				943,771	456,649	$ 239,758
Net income	$ 30,542	$ 138,423	$ (53,699)	$ (48,408)	$ (47,590)	$ (50,711)	$ (38,380)	$ (28,300)	$ 66,858	$ (164,981)	$ (68,357)