Business Segments (Identifiable Assets by Segment) (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	$ 41,345,792	$ 40,448,635
Operating Segments | Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	33,246,341	32,911,507
Operating Segments | Reinsurance and Monoline Excess Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	5,667,553	5,669,729
Corporate Reconciling Items And Eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	$ 2,431,898	$ 1,867,399