Income taxes - Summary Of Changes In Unrecognized Tax Positions (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	$ 2,608	$ 2,608
Decrease related to prior year tax positions	(2,308)	0
Balance at end of year	$ 300	$ 2,608