Income taxes - Schedule of net deferred tax liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net deferred tax liabilities
Operating loss carry-forwards	$ 58,185	$ 82,292	$ 89,513
Pensions	95	255	63
Share-based compensation	703	3,617	2,978
Other	8,184	8,283	8,952
Less: Valuation allowance	(49,337)	(70,241)	(69,928)
Net deferred tax assets	17,830	24,206	31,578
Other	(4,817)	(5,032)	(7,427)
Property, plant and equipment	(128,206)	(141,231)	(147,265)
Deferred tax liabilities	(133,023)	(146,263)	(154,692)
Net deferred tax liabilities	$ (115,193)	$ (122,057)	$ (123,114)