Industry Segments (Identifiable Assets by Segment) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	$ 37,202,015	$ 33,861,099
Revenues	12,142,938	11,166,499	$ 9,455,466
Operating Segments | Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	30,132,743	27,009,652
Revenues	9,961,152	8,952,493	7,578,592
Operating Segments | Insurance | Foreign countries
Segment Reporting, Asset Reconciling Item [Line Items]
Revenues	1,171,000	1,029,000	873,000
Operating Segments | Reinsurance & Monoline Excess
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	5,389,125	5,195,752
Revenues	1,481,991	1,386,639	1,203,647
Operating Segments | Reinsurance & Monoline Excess | Foreign countries
Segment Reporting, Asset Reconciling Item [Line Items]
Revenues	463,000	412,000	380,000
Corporate, other and eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	1,680,147	1,655,695
Revenues	$ 652,753	$ 624,970	$ 582,595