Taxation (Income Before Income Taxes, Foreign and Domestic) (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 Expense (Benefit)
(Loss) income before taxes	$ 43,425	$ (722,471)	$ 218,254	$ 132,608	$ 83,550	$ 186,745	$ 179,168	$ 180,925	$ (328,184)	$ 630,388	$ 496,376
Domestic
Income Tax Expense (Benefit)
Domestic									(262,827)	652,758	511,114
U.K.
Income Tax Expense (Benefit)
Foreign									(41,656)	(24,278)	(22,712)
Singapore
Income Tax Expense (Benefit)
Foreign									(12,421)	2,180	(4,737)
U.S.
Income Tax Expense (Benefit)
Foreign									(11,897)	(1,236)	12,523
Ireland
Income Tax Expense (Benefit)
Foreign									$ 617	$ 964	$ 188