Business Segments (Identifiable Assets by Segment) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	$ 24,587,832	$ 24,299,917
Operating Segments | Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	19,322,526	19,263,193
Operating Segments | Reinsurance
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	3,093,808	3,169,731
Corporate, other and eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	$ 2,171,498	$ 1,866,993