Business Segments (Identifiable Assets by Segment) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	$ 24,336,076	$ 23,350,076
Operating Segments | Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	19,136,776	19,137,758
Operating Segments | Reinsurance
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	3,243,610	2,524,338
Corporate, other and eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	$ 1,955,690	$ 1,687,980