Business Segments (Identifiable Assets by Segment) (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	$ 32,250,954	$ 32,047,876
Operating Segments | Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	25,057,202	24,403,918
Operating Segments | Reinsurance & Monoline Excess
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	5,058,976	4,917,985
Corporate, other and eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Consolidated Assets	$ 2,134,776	$ 2,725,973