Business Segments (Identifiable Assets by Segment) (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	$ 42,658,057	$ 40,448,635
Operating Segments | Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	34,532,276	32,911,507
Operating Segments | Reinsurance and Monoline Excess Segment
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	5,799,205	5,669,729
Corporate Reconciling Items And Eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	$ 2,326,576	$ 1,867,399