Background and Organization - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Net loss	$ (76,396)	$ (72,176)	$ (50,636)
Net cash used from operations	(122,194)	(70,053)	$ (28,156)
Cash and cash equivalents	$ 167,432	$ 57,389