Taxation (Income (Loss) Before Income Taxes, Foreign and Domestic) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Expense (Benefit)
Income (loss) before taxes	$ 36,885	$ 111,608	$ 458,330	$ 360,659	$ (132,838)	$ 50,280	$ 256,373	$ 88,800	$ 967,482	$ 262,615	$ (328,184)
Bermuda
Income Tax Expense (Benefit)
Domestic									861,068	349,959	(262,827)
U.S.
Income Tax Expense (Benefit)
Foreign									102,724	(56,261)	(11,897)
Australia
Income Tax Expense (Benefit)
Foreign									3,390	0	0
Switzerland
Income Tax Expense (Benefit)
Foreign									14,255	166	0
Ireland
Income Tax Expense (Benefit)
Foreign									(388)	551	617
Singapore
Income Tax Expense (Benefit)
Foreign									(6,334)	(3,226)	(12,421)
U.K.
Income Tax Expense (Benefit)
Foreign									$ (7,233)	$ (28,574)	$ (41,656)