Business Segments (Identifiable Assets by Segment) (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	$ 34,296,064	$ 33,815,103
Operating Segments | Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	27,062,484	27,012,479
Operating Segments | Reinsurance & Monoline Excess
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	5,191,920	5,195,752
Corporate Reconciling Items And Eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	$ 2,041,660	$ 1,606,872