Income taxes - Reconciliation of Beginning and Ending Amount 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]
Beginning Balance	$ 26,196	$ 25,539	$ 23,606
Gross increases for tax positions of current year		657	1,933
Net changes in tax positions in current year	(5,544)
Ending Balance	$ 20,652	$ 26,196	$ 25,539