Statutory Financial Information (Statutory Net Income (Loss)) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
United States [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	$ 419,396	$ 414,957	$ 312,828
United Kingdom [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	108,759	40,203	(25,785)
Bermuda [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	447,479	(131,411)	(78,070)
Other [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	$ (360)	$ (5,193)	$ (4,812)