Basis of presentation and summary of significant accounting policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)
Significant Accounting Policies [Line Items]
Operating segment | Segment	1
Total Assets	$ 110,329,000	$ 154,380,000
Accumulated deficit	198,502,000	148,278,000
United States [Member]
Significant Accounting Policies [Line Items]
Total Assets	$ 96,230	$ 149,796
Minimum [Member]
Significant Accounting Policies [Line Items]
Estimated useful lives of property and equipment	3 years
Maximum [Member]
Significant Accounting Policies [Line Items]
Estimated useful lives of property and equipment	5 years