Condensed summary of significant accounting policies - Additional Information (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2015 USD ($) Segment	Dec. 31, 2014 USD ($)
Significant Accounting Policies [Line Items]
Operating segment | Segment	1
Total Assets	$ 118,028	$ 41,422
Accumulated deficit	(105,613)	(89,416)
United States [Member]
Significant Accounting Policies [Line Items]
Total Assets	$ 112,162	$ 31,099