Statutory Requirements (Statutory Net Income (Loss)) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Bermuda
Statutory Accounting Practices [Line Items]
Statutory Net Income (Loss)	$ 657,182	$ 326,386	$ (334,142)
Switzerland
Statutory Accounting Practices [Line Items]
Statutory Net Income (Loss)	(14,679)	0	0
U.S.
Statutory Accounting Practices [Line Items]
Statutory Net Income (Loss)	37,827	25,851	(3,627)
U.K.
Statutory Accounting Practices [Line Items]
Statutory Net Income (Loss)	$ (666,595)	$ (6,692)	$ (57,050)