Income taxes - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Tax Credit Carryforward [Line Items]
Total income tax benefit resulting from the 2017 Act	$ 7,800
Income tax expense impact on deemed repatriation of deferred foreign income	1,200
Net deferred tax liabilities	14,848	$ 3,996
Net increase (decrease) in valuation allowance	(200)	200
Deferred tax assets relating to NOL carryforwards	6,483	6,275
U.S. Federal
Tax Credit Carryforward [Line Items]
Total income tax benefit resulting from the 2017 Act	7,200
Net deferred tax liabilities	7,700	1,200
State
Tax Credit Carryforward [Line Items]
Total income tax benefit resulting from the 2017 Act	600
Foreign
Tax Credit Carryforward [Line Items]
Net deferred tax liabilities	$ 7,100	$ 2,800