Income taxes - Reconciliation of Income Tax Expenses Computed at Federal Statutory Rate with Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Expected income tax expense at 21% in 2019 and 2018 and 35% in 2017	$ 22,116	$ 12,816	$ 22,253
Nondeductible expenses	3,249	1,872	2,610
Valuation allowance	1,326	1,741	0
Net expense (benefit) for the Canadian branch	613	128	(1,480)
Return-to-provision and true-up adjustments	(776)	(370)	923
Research and development credits	(278)	(732)	(2,158)
2017 Act impact from the U.S. corporate tax rate change	0	(745)	(7,196)
2017 Act impact from deemed repatriation of deferred foreign income	0	(624)	1,213
Other – net	445	(579)	(1,244)
Total income tax expense	$ 26,695	$ 13,507	$ 14,921
Effective income tax rate	25.30%	22.10%	23.50%