Income taxes - Reconciliation of Gross Amounts of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Gross unrecognized tax benefits, Beginning Balance	$ 14,400	$ 17,100	$ 17,700
Increases in prior years’ unrecognized tax benefits	0	0	700
Decreases in prior years’ unrecognized tax benefits	(1,300)	(700)	(2,400)
Increases in current year's unrecognized tax benefits	1,300	1,200	1,600
Decreases in unrecognized tax benefits from the lapse of statutes of limitations	(2,300)	(2,600)	(300)
Settlements	100
Settlements		(600)	(200)
Gross unrecognized tax benefits, Ending Balance	$ 12,200	$ 14,400	$ 17,100