Income taxes - Provision for income taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax [Line Items]
Difference in taxation rates	$ 9,509	$ 12,948	$ 15,089
Pre-tax (loss)/income	40,756	25,354	8,105
Provision for income taxes, Current	16,673	17,164	26,489
Provision for income taxes, Deferred	(305)	(123)	(10,947)
Provision for income taxes	16,368	17,041	15,542
UK
Income Tax [Line Items]
Pre-tax (loss)/income	(6,702)	(18,965)	(3,476)
Provision for income taxes, Current	949	1,585	535
Provision for income taxes, Deferred	(444)	(2,262)	(1,279)
Provision for income taxes	505	(677)	(744)
Bermuda
Income Tax [Line Items]
Pre-tax (loss)/income	160	(4,315)	(25,702)
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 [Line Items]
Pre-tax (loss)/income	2,681	1,439	312
Provision for income taxes, Current	(1,510)	1,310	1,867
Provision for income taxes, Deferred	(1,978)	1,698	(193)
Provision for income taxes	(3,488)	3,008	1,674
Brazil
Income Tax [Line Items]
Pre-tax (loss)/income	15,303	10,380	21,275
Provision for income taxes, Current	5,498	2,768	7,872
Provision for income taxes, Deferred	(121)	760	(447)
Provision for income taxes	5,377	3,528	7,425
Italy
Income Tax [Line Items]
Pre-tax (loss)/income	19,971	17,351	9,572
Provision for income taxes, Current	4,468	4,006	6,752
Provision for income taxes, Deferred	1,998	(824)	(1,738)
Provision for income taxes	6,466	3,182	5,014
Peru
Income Tax [Line Items]
Pre-tax (loss)/income	13,965	12,078	9,505
Provision for income taxes, Current	4,261	3,154	3,923
Provision for income taxes, Deferred	1,625	(677)	(1,652)
Provision for income taxes	5,886	2,477	2,271
Rest of the world
Income Tax [Line Items]
Pre-tax (loss)/income	(4,622)	7,386	(3,381)
Provision for income taxes, Current	3,007	4,341	5,540
Provision for income taxes, Deferred	(1,385)	1,182	(5,638)
Provision for income taxes	$ 1,622	$ 5,523	$ (98)