Background and Organization - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Net loss	$ (20,831)	$ (32,852)	$ (84,990)	$ (59,656)
Net cash used from operating activities			(76,632)	$ (77,429)
Cash and cash equivalents	$ 90,756		$ 90,756		$ 167,432