Background and Organization - Additional Information (Detail) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2017 USD ($) Agreement	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Background And Organization [Line Items]
Net loss		$ (11,216)	$ (14,146)	$ (47,530)	$ (39,629)
Net cash used from operations				(43,694)	(28,070)
Cash and cash equivalents		$ 34,413	$ 14,607	$ 34,413	$ 14,607	$ 14,198	$ 20,667
2017 Private Placement | Subsequent Event
Background And Organization [Line Items]
Proceeds from private placement of common stock	$ 50,000
Number of securities purchase agreements | Agreement	2