Statutory Requirements (Statutory Capital and Surplus) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Bermuda
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	$ 4,212,787	$ 4,878,811
Required statutory capital and surplus	807,108	1,782,778
Restricted net assets	1,779,319	838,633
U.S.
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	523,340	521,522
Required statutory capital and surplus	221,023	219,164
Restricted net assets	324,567	321,362
U.K.
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	491,213	485,256
Required statutory capital and surplus	491,213	485,256
Restricted net assets	$ 0	$ 0