Income taxes - Reconciliation of tax charge at statutory tax rate to provision for income taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
(Losses)/earnings before provision for income taxes and earnings from unconsolidated companies, net of tax	$ (23,614)	$ (28,303)	$ 40,756
Tax (benefit)/charge at statutory tax rate of 19%, 19.25% and 20%	(4,487)	(5,448)	8,151
Exchange rate movements on deferred tax	657	2,400	(1,785)
Notional interest deductions	(1,289)	(960)	(1,812)
Imputed cross border charges	815	763	995
Disallowable goodwill impairment charges	1,093	0	0
Other permanent disallowable expenditure	5,159	3,664	483
Change in valuation allowance	5,547	2,215	4,876
Difference in taxation rates	8,316	13,874	9,509
(Reduction in)/additional deferred tax liability in respect of VIE	(1,732)	7,263	0
Change in provisions for uncertain tax positions	46	160	(3,350)
Change in tax rates	0	(19,807)	(643)
Transition tax in U.S.	(190)	2,330	0
Other	48	100	(56)
Provision for income taxes	$ 13,983	$ 6,554	$ 16,368
Statutory tax rate, percentage	19.00%	19.25%	20.00%