Background and Organization - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Net loss	$ (4,814)	$ (27,961)
Net cash used from operations	(37,454)	$ (11,964)
Cash and cash equivalents	$ 78,916		$ 57,389