Income taxes - Summary of (Benefit from) Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current
State			$ 1
Foreign		$ 60
Deferred
Federal	$ (588)		(9,390)
State	(24)		(2,408)
Total income tax expense (benefit)	$ (612)	$ 60	$ (11,797)