Taxation (Income Before Income Taxes, Foreign and Domestic) (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 Expense (Benefit)
Income before taxes	$ 83,550	$ 186,745	$ 179,168	$ 180,925	$ 134,322	$ 107,704	$ 89,154	$ 165,196	$ 630,388	$ 496,376	$ 686,864
Bermuda | Domestic Country
Income Tax Expense (Benefit)
Domestic									652,758	511,114	701,476
U.K. | Foreign
Income Tax Expense (Benefit)
Foreign									(24,278)	(22,712)	(3,166)
U.S. | Internal Revenue Service (IRS)
Income Tax Expense (Benefit)
Foreign									(1,236)	12,523	(10,977)
Ireland | Foreign
Income Tax Expense (Benefit)
Foreign									964	188	1,549
Singapore | Foreign
Income Tax Expense (Benefit)
Foreign									$ 2,180	$ (4,737)	$ (2,018)