Accumulated other comprehensive loss - Schedule of accumulated other comprehensive income/(loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ (334,542)
Ending Balance	(346,777)	$ (334,542)
OCI foreign currency translation adjustments before reclassification, tax	(809)	0
OCI pension liability before reclassification, tax	(530)	147
OCI derivative financial instruments before reclassification, tax	(889)	(1,310)
OCI foreign currency translation adjustments reclassified, tax	0	0
OCI derivative financial instruments reclassified, tax	1,111	958
OCI pension liability before reclassified, tax	0	0
Accumulated other comprehensive income/(loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(334,542)	(246,420)
Other comprehensive loss before reclassifications, net of tax benefit of $(809), $(889) and $(530)	(15,044)	(91,071)
Amounts reclassified from AOCI, net of tax provision of $Nil, $1,111 and $Nil	2,809	2,949
Net current period other comprehensive (loss)/income	(12,235)	(88,122)
Ending Balance	(346,777)	(334,542)
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(320,508)	(232,328)
Other comprehensive loss before reclassifications, net of tax benefit of $(809), $(889) and $(530)	(10,983)	(88,180)
Amounts reclassified from AOCI, net of tax provision of $Nil, $1,111 and $Nil	0	0
Net current period other comprehensive (loss)/income	(10,983)	(88,180)
Ending Balance	(331,491)	(320,508)
Derivative financial instruments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(4,091)	(3,569)
Other comprehensive loss before reclassifications, net of tax benefit of $(809), $(889) and $(530)	(1,942)	(3,471)
Amounts reclassified from AOCI, net of tax provision of $Nil, $1,111 and $Nil	2,809	2,949
Net current period other comprehensive (loss)/income	867	(522)
Ending Balance	(3,224)	(4,091)
Pension liability
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(9,943)	(10,523)
Other comprehensive loss before reclassifications, net of tax benefit of $(809), $(889) and $(530)	(2,119)	580
Amounts reclassified from AOCI, net of tax provision of $Nil, $1,111 and $Nil	0	0
Net current period other comprehensive (loss)/income	(2,119)	580
Ending Balance	$ (12,062)	$ (9,943)