Income taxes - Provision for income taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax
Pre-tax (loss)/income	$ (23,614)	$ (28,303)	$ 40,756
Provision for income taxes, Current	20,206	20,196	16,673
Provision for income taxes, Deferred	(6,223)	(13,642)	(305)
Provision for income taxes	13,983	6,554	16,368
UK
Income Tax
Pre-tax (loss)/income	(25,025)	(18,518)	(6,702)
Provision for income taxes, Current	3,893	1,671	949
Provision for income taxes, Deferred	0	(372)	(444)
Provision for income taxes	3,893	1,299	505
Bermuda
Income Tax
Pre-tax (loss)/income	(7,161)	(34,873)	160
Provision for income taxes, Current	0	0	0
Provision for income taxes, Deferred	0	0	0
Provision for income taxes	0	0	0
United States
Income Tax
Pre-tax (loss)/income	4	(1,850)	2,681
Provision for income taxes, Current	832	3,332	(1,510)
Provision for income taxes, Deferred	(1,789)	(12,987)	(1,978)
Provision for income taxes	(957)	(9,655)	(3,488)
Brazil
Income Tax
Pre-tax (loss)/income	3,863	2,193	15,303
Provision for income taxes, Current	1,810	2,322	5,498
Provision for income taxes, Deferred	51	(804)	(121)
Provision for income taxes	1,861	1,518	5,377
Italy
Income Tax
Pre-tax (loss)/income	15,748	20,265	19,971
Provision for income taxes, Current	5,701	5,359	4,468
Provision for income taxes, Deferred	(7,328)	972	1,998
Provision for income taxes	(1,627)	6,331	6,466
Peru
Income Tax
Pre-tax (loss)/income	14,876	14,367	13,965
Provision for income taxes, Current	5,044	4,712	4,261
Provision for income taxes, Deferred	(298)	248	1,625
Provision for income taxes	4,746	4,960	5,886
Rest of the world
Income Tax
Pre-tax (loss)/income	(25,919)	(9,887)	(4,622)
Provision for income taxes, Current	2,926	2,800	3,007
Provision for income taxes, Deferred	3,141	(699)	(1,385)
Provision for income taxes	$ 6,067	$ 2,101	$ 1,622