Summary of significant accounting policies - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 01, 2019
Significant Accounting Policies [Line Items]
Recognition of right-of-use assets	$ 170
Recognition of lease liabilities	$ 405
Accounting Standards Update 2016-02 [Member]
Significant Accounting Policies [Line Items]
Recognition of right-of-use assets		$ 200
Recognition of lease liabilities		500
Derecognition of deferred rent liability		$ 300