Background and Organization - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 28, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Background And Organization [Line Items]
Net loss		$ (72,176)	$ (50,636)	$ (44,995)
Net cash used from operations		(70,053)	(28,156)	(39,849)
Cash and cash equivalents		$ 57,389	$ 14,198	$ 20,667	$ 11,129
Subsequent Event
Background And Organization [Line Items]
Proceeds from offering of common stock, gross	$ 59,100