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, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Expected income tax expense (benefit) at 35%	$ 22,253	$ 26,279	$ (194)
2017 Act impact from the U.S. corporate tax rate change	(7,196)	0	0
Nondeductible expenses	2,610	2,772	2,768
Research and development credits	(2,158)	(3,434)	0
Federal net benefit offset to Canadian expense	(1,480)	(639)	0
2017 Act impact from deemed repatriation of deferred foreign income	1,213	0	0
Return-to-provision and true-up adjustments	923	(4,127)	(1,329)
Impairment of goodwill	0	0	7,099
Other – net	(1,244)	(1,246)	(2,694)
Total income tax expense	$ 14,921	$ 19,605	$ 5,650
Effective income tax rate	23.50%	26.10%	(1019.40%)