Accumulated other comprehensive income/loss - Changes in accumulated other comprehensive income/(loss) by component (net of tax) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Increase (Decrease) in Stockholders' Equity
Balance at January 1, 2017			$ 686,832	$ 658,425
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [Abstract]
Total other comprehensive income/(losses), net of tax	$ 16,151	$ (9,290)	30,052	4,449
Balance	697,493	670,715	697,493	670,715
Accumulated other comprehensive income/(loss)
Increase (Decrease) in Stockholders' Equity
Balance at January 1, 2017			(352,339)	(334,542)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [Abstract]
Other comprehensive income before reclassifications, net of tax (benefit)/provision of $Nil, $(56) and $65			29,064
Amounts reclassified from AOCI, net of tax provision of $Nil, $251 and $Nil			979
Total other comprehensive income/(losses), net of tax			30,043	4,347
Balance	(322,296)	$ (330,195)	(322,296)	$ (330,195)
Foreign currency translation adjustments
Increase (Decrease) in Stockholders' Equity
Balance at January 1, 2017			(337,053)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [Abstract]
Other comprehensive income before reclassifications, net of tax (benefit)/provision of $Nil, $(56) and $65			28,968
Amounts reclassified from AOCI, net of tax provision of $Nil, $251 and $Nil			0
Total other comprehensive income/(losses), net of tax			28,968
Balance	(308,085)		(308,085)
Other Comprehensive Income (Loss) before Reclassifications Tax
Other Comprehensive Income (Loss) before Reclassifications, Tax			0
Reclassification from AOCI, Current Period, Tax
Reclassification from AOCI, Current Period, Tax			0
Derivative financial instruments
Increase (Decrease) in Stockholders' Equity
Balance at January 1, 2017			(3,224)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [Abstract]
Other comprehensive income before reclassifications, net of tax (benefit)/provision of $Nil, $(56) and $65			(220)
Amounts reclassified from AOCI, net of tax provision of $Nil, $251 and $Nil			979
Total other comprehensive income/(losses), net of tax			759
Balance	(2,465)		(2,465)
Other Comprehensive Income (Loss) before Reclassifications Tax
Other Comprehensive Income (Loss) before Reclassifications, Tax			(56)
Reclassification from AOCI, Current Period, Tax
Reclassification from AOCI, Current Period, Tax			251
Pension liability
Increase (Decrease) in Stockholders' Equity
Balance at January 1, 2017			(12,062)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [Abstract]
Other comprehensive income before reclassifications, net of tax (benefit)/provision of $Nil, $(56) and $65			316
Amounts reclassified from AOCI, net of tax provision of $Nil, $251 and $Nil			0
Total other comprehensive income/(losses), net of tax			316
Balance	$ (11,746)		(11,746)
Other Comprehensive Income (Loss) before Reclassifications Tax
Other Comprehensive Income (Loss) before Reclassifications, Tax			65
Reclassification from AOCI, Current Period, Tax
Reclassification from AOCI, Current Period, Tax			$ 0