Income taxes - Schedule of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	$ 764	$ 1,208	$ 1,256
Increases for prior year tax positions	0	63	438
Increases for current year tax positions	173	68	103
Decreases for prior year tax positions	(8)	(1)	(589)
Decreases due to settlements	0	(32)	0
Decreases due to statutes lapsing	(358)	(542)	0
Balance at end of year	$ 571	$ 764	$ 1,208