Business Segments (Identifiable Assets by Segment) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	$ 23,993,491	$ 23,350,076
Operating Segments | Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	18,687,170	19,137,758
Operating Segments | Reinsurance
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	3,085,961	2,524,338
Corporate, other and eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	$ 2,220,360	$ 1,687,980