Income taxes - Schedule of net deferred tax liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net deferred tax liabilities
Operating loss carry-forwards	$ 54,465	$ 58,185	$ 82,292
Pensions	0	95	255
Stock options	680	703	3,617
Other	9,226	8,184	8,283
Less: Valuation allowance	(50,951)	(49,337)	(70,241)
Net deferred tax assets	13,420	17,830	24,206
Other	(6,763)	(4,817)	(5,032)
Property, plant and equipment	(110,609)	(128,206)	(141,231)
Deferred tax liabilities	(117,372)	(133,023)	(146,263)
Net deferred tax liabilities	$ (103,952)	$ (115,193)	$ (122,057)