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, 2020	Dec. 31, 2019	Dec. 31, 2018
Effective Tax Rate Reconciliation [Line Items]
Expected income tax expense at 21%	$ 42,785	$ 22,116	$ 12,816
State income tax expense (benefit) - net of Federal impact	2,944	818	(5)
Valuation allowance	2,427	1,326	1,741
Nondeductible expenses	2,399	3,249	1,872
Effective Income Tax Rate Reconciliation, Tax Credit, Research, Amount	(1,055)	(278)	(732)
Return-to-provision and true-up adjustments	(438)	(776)	(370)
2017 Act impact from the U.S. corporate tax rate change	0	0	(745)
2017 Act impact from deemed repatriation of deferred foreign income	0	0	(624)
Other – net	(216)	(511)	(871)
Total income tax expense	$ 48,833	$ 26,695	$ 13,507
Effective income tax rate	24.00%	25.30%	22.10%
Foreign
Effective Tax Rate Reconciliation [Line Items]
Foreign income tax rate differential	$ 1,465	$ 138	$ 297
Canada
Effective Tax Rate Reconciliation [Line Items]
Foreign income tax rate differential	$ (1,478)	$ 613	$ 128