Background and Organization - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Net loss	$ (21,990)	$ (8,353)	$ (26,804)	$ (36,314)
Net cash used from operations			(48,838)	$ (23,883)
Cash and cash equivalents	$ 66,143		$ 66,143		$ 57,389