Basis of presentation and summary of significant accounting policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)
Significant Accounting Policies [Line Items]
Operating segment | Segment	1
Total Assets	$ 77,618,000	$ 110,329,000
Accumulated deficit	(230,087,000)	(198,502,000)
Accounting Standards Update 2016-02 [Member]
Significant Accounting Policies [Line Items]
Recognition of right-of-use assets	200,000
Recognition lease liabilities	500,000
Derecognition of deferred rent liability	300,000
United States [Member]
Significant Accounting Policies [Line Items]
Total Assets	$ 52,247	$ 96,230
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