Basis of presentation and summary of significant accounting policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015 USD ($) Segment	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Significant Accounting Policies [Line Items]
Foreign exchange loss and others	$ (580,000)	$ (549,000)	$ (41,000)
Effect of exchange rate changes on cash and cash equivalents	(527,000)	(320,000)	(116,000)
Impairment charges of property and equipment	$ 0	0
Percentage of tax benefit upon settlement	50.00%
Operating segment | Segment	1
Total Assets	$ 113,343,000	41,422,000
Accumulated deficit	(111,276,000)	(89,416,000)
Reclassifications [Member]
Significant Accounting Policies [Line Items]
Foreign exchange loss and others		320,000	$ 116,000
United States [Member]
Significant Accounting Policies [Line Items]
Total Assets	$ 111,366,000	$ 33,102,000
Maximum [Member]
Significant Accounting Policies [Line Items]
Estimated useful lives of property and equipment	5 years