Industry Segments (Identifiable Assets by Segment) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	$ 33,861,099	$ 32,086,414
Revenues	11,166,499	9,455,466	$ 8,098,925
Operating Segments | Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	27,009,652	24,414,305
Revenues	8,952,493	7,578,592	6,478,834
Operating Segments | Insurance | Foreign countries
Segment Reporting, Asset Reconciling Item [Line Items]
Revenues	1,029,000	873,000	692,000
Operating Segments | Reinsurance & Monoline Excess
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	5,195,752	4,916,894
Revenues	1,386,639	1,203,647	1,009,203
Operating Segments | Reinsurance & Monoline Excess | Foreign countries
Segment Reporting, Asset Reconciling Item [Line Items]
Revenues	412,000	380,000	292,000
Corporate, other and eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	1,655,695	2,755,215
Revenues	$ 624,970	$ 582,595	$ 507,888