Business Segments (Identifiable Assets by Segment) (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	$ 36,111,348	$ 33,815,103
Operating Segments | Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	28,862,050	27,012,479
Operating Segments | Reinsurance & Monoline Excess
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	5,194,783	5,195,752
Corporate Reconciling Items And Eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	$ 2,054,515	$ 1,606,872