Basis of presentation and summary of significant accounting policies - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) Segment	Dec. 31, 2015 USD ($)
Significant Accounting Policies [Line Items]
Operating segment | Segment	1
Total Assets	$ 154,380	$ 113,343
Accumulated deficit	$ 148,278	111,276
Maximum [Member]
Significant Accounting Policies [Line Items]
Estimated useful lives of property and equipment	5 years
United States [Member]
Significant Accounting Policies [Line Items]
Total Assets	$ 149,796	$ 111,366