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, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Expected income tax expense at 21% in 2018 and 35% in 2017 and 2016	$ 12,816	$ 22,253	$ 26,279
Nondeductible expenses	1,872	2,610	2,772
Valuation allowance	1,741	0	0
2017 Act impact from the U.S. corporate tax rate change	(745)	(7,196)	0
Research and development credits	(732)	(2,158)	(3,434)
2017 Act impact from deemed repatriation of deferred foreign income	(624)	1,213	0
Return-to-provision and true-up adjustments	(370)	923	(4,127)
Net expense (benefit) for the Canadian branch	128	(1,480)	(639)
Other – net	(579)	(1,244)	(1,246)
Total income tax expense	$ 13,507	$ 14,921	$ 19,605
Effective income tax rate	22.10%	23.50%	26.10%