Shareholders' equity - Comprehensive Income (Loss) - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period	$ (12,496)	$ (32,970)	$ (28,576)
Other Comprehensive Income (Loss), Net of Tax	(12,154)	20,329	(4,394)
Amounts reclassified from accumulative other comprehensive income, net of tax		145
Balance, end of period	(24,650)	(12,496)	(32,970)	$ (28,576)
Reclassification from AOCI, tax expense (benefit)		0	0
Accumulated Foreign Currency Adjustment Attributable to Parent [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period	(10,054)	(31,472)	(27,552)
Other Comprehensive Income (Loss), Net of Tax	(12,911)	21,273	(3,920)
Amounts reclassified from accumulative other comprehensive income, net of tax		145
Balance, end of period	(22,965)	(10,054)	(31,472)	(27,552)
Other comprehensive income (loss) before reclassification, tax expense (benefit)	0	0	222
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period	(2,442)	(1,498)
Other Comprehensive Income (Loss), Net of Tax	757	(944)	(474)	$ (1,024)
Amounts reclassified from accumulative other comprehensive income, net of tax		0
Balance, end of period	(1,685)	(2,442)	(1,498)
Other comprehensive income (loss) before reclassification, tax expense (benefit)	$ (59)	$ (37)	$ (87)