Accumulated other comprehensive loss - Schedule of accumulated other comprehensive income/(loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss), Net of Tax
Beginning balance	$ 698,836	$ 686,832	$ 658,425
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Amounts reclassified from AOCI, net of tax provision of $Nil, $Nil and $Nil	1,759	2,733
Total other comprehensive (losses)/income, net of tax	(36,642)	50,984	(12,100)
Ending balance	640,261	698,836	686,832
Accumulated other comprehensive income/(loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax
Beginning balance	(301,322)	(352,339)	(340,104)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Other comprehensive (loss)/income before reclassifications, net of tax provision of $Nil, $Nil and $Nil	(38,656)	48,284
Amounts reclassified from AOCI, net of tax provision of $Nil, $Nil and $Nil	1,759	2,733
Total other comprehensive (losses)/income, net of tax	(36,897)	51,017	(12,235)
Ending balance	(338,219)	(301,322)	(352,339)
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax
Beginning balance	(288,266)	(337,053)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Other comprehensive (loss)/income before reclassifications, net of tax provision of $Nil, $Nil and $Nil	(39,875)	48,095
Amounts reclassified from AOCI, net of tax provision of $Nil, $Nil and $Nil	0	692
Total other comprehensive (losses)/income, net of tax	(39,875)	48,787
Ending balance	(328,141)	(288,266)	(337,053)
Other Comprehensive Income (Loss) before Reclassifications, Tax
Other comprehensive income before reclassifications, tax	0	0
Reclassification from AOCI, Current Period, Tax
Amounts reclassified from AOCI, tax	0	0
Derivative financial instruments
Accumulated Other Comprehensive Income (Loss), Net of Tax
Beginning balance	(1,304)	(3,224)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Other comprehensive (loss)/income before reclassifications, net of tax provision of $Nil, $Nil and $Nil	530	(121)
Amounts reclassified from AOCI, net of tax provision of $Nil, $Nil and $Nil	1,759	2,041
Total other comprehensive (losses)/income, net of tax	2,289	1,920
Ending balance	985	(1,304)	(3,224)
Other Comprehensive Income (Loss) before Reclassifications, Tax
Other comprehensive income before reclassifications, tax	0	(50)
Reclassification from AOCI, Current Period, Tax
Amounts reclassified from AOCI, tax	0	860
Pension liability
Accumulated Other Comprehensive Income (Loss), Net of Tax
Beginning balance	(11,752)	(12,062)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Other comprehensive (loss)/income before reclassifications, net of tax provision of $Nil, $Nil and $Nil	689	310
Amounts reclassified from AOCI, net of tax provision of $Nil, $Nil and $Nil	0	0
Total other comprehensive (losses)/income, net of tax	689	310
Ending balance	(11,063)	(11,752)	$ (12,062)
Other Comprehensive Income (Loss) before Reclassifications, Tax
Other comprehensive income before reclassifications, tax	0	73
Reclassification from AOCI, Current Period, Tax
Amounts reclassified from AOCI, tax	$ 0	$ 0