Taxation - Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
(Loss) income before income taxes									$ (173,303)	$ 49,712	$ 126,322
Less: income tax (benefit) expense									(3,558)	1,574	2,038
Net (loss) income	$ (124,857)	$ (55,054)	$ (16,335)	$ 26,501	$ (69,374)	$ 40,796	$ 39,887	$ 36,829	$ (169,745)	$ 48,138	$ 124,284
Reconciliation of effective tax rate (% of income before income taxes)
Bermuda tax rate									0.00%	0.00%	0.00%
U.S. taxes at statutory rates									2.80%	(10.80%)	(2.20%)
Rate change in the U.S.									2.90%	0.00%	0.00%
Valuation allowance in respect of U.S. taxes									(3.40%)	13.20%	3.20%
Other jurisdictions									(0.20%)	0.80%	0.60%
Actual tax rate									2.10%	3.20%	1.60%