Income taxes - Reconciliation of Gross Amounts of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Gross unrecognized tax benefits, Beginning Balance	$ 12,200	$ 14,400	$ 17,100
Increases in prior years’ unrecognized tax benefits	91	0	0
Decreases in prior years’ unrecognized tax benefits	(464)	(1,300)	(700)
Increases in current year's unrecognized tax benefits	1,569	1,300	1,200
Decreases in unrecognized tax benefits from the lapse of statutes of limitations	(1,866)	(2,300)	(2,600)
Settlements	(300)		(600)
Settlements		100
Gross unrecognized tax benefits, Ending Balance	$ 11,230	$ 12,200	$ 14,400