Statutory Financial Information (Statutory Net Income (Loss)) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
United States [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	$ 414,957	$ 312,828	$ 249,176
United Kingdom [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	40,203	(25,785)	78,033
Bermuda [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	(131,411)	(78,070)	132,442
Other Countries [Member]
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	$ (5,193)	$ (4,812)	$ (965)