Statutory Requirements (Statutory Net Income (Loss)) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Bermuda
Statutory Accounting Practices [Line Items]
Statutory Net Income (Loss)	$ 326,386	$ (334,142)	$ 625,371
U.S.
Statutory Accounting Practices [Line Items]
Statutory Net Income (Loss)	25,851	(3,627)	43,292
U.K.
Statutory Accounting Practices [Line Items]
Statutory Net Income (Loss)	$ (6,692)	$ (57,050)	$ 28,007