Statutory Requirements (Statutory Net Income (Loss)) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Bermuda
Statutory Accounting Practices [Line Items]
Statutory net income	$ 625,371	$ 355,132	$ 623,931
U.S.
Statutory Accounting Practices [Line Items]
Statutory net income	43,292	58,752	0
U.K.
Statutory Accounting Practices [Line Items]
Statutory net income	$ 28,007	$ 1,627	$ 24,433