Income taxes (Reconciliation of effective tax rate) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Bermuda tax rate									0.00%	0.00%	0.00%
Tax charge by jurisdiction
Expected tax expense at Bermuda statutory rate of 0%									$ 0	$ 0	$ 0
Foreign tax rate differential									11,394	6,462	7,993
Changes in valuation allowance									(36,990)	9,830	(7,284)
Tax exempt income and expenses not deductible									(1,960)	393	(6,055)
Share compensation tax windfall									1,203	906	3,513
Impact of enacted changes in tax rates									8,931	69	250
Prior years tax adjustments									(5,199)	(12,272)	666
Actual income tax (benefit) expense									2,892	988	1,072
Total income tax (benefit) expense	$ (21,147)	$ 1,830	$ 1,706	$ (2,118)	$ (756)	$ 2,018	$ 2,549	$ 2,565	$ (19,729)	$ 6,376	$ 155