Industry Segments (Identifiable Assets by Segment) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	$ 40,567,268	$ 37,202,015
Revenues	13,638,752	12,142,938	$ 11,166,499
Operating Segments | Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	33,030,140	29,976,619
Revenues	11,181,501	9,827,866	8,749,019
Operating Segments | Insurance | Foreign countries
Segment Reporting, Asset Reconciling Item [Line Items]
Revenues	1,471,000	1,171,000	1,029,000
Operating Segments | Reinsurance & Monoline Excess
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	5,669,729	5,545,249
Revenues	1,696,905	1,615,277	1,590,113
Operating Segments | Reinsurance & Monoline Excess | Foreign countries
Segment Reporting, Asset Reconciling Item [Line Items]
Revenues	485,000	463,000	412,000
Corporate, other and eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	1,867,399	1,680,147
Revenues	$ 642,638	$ 652,753	$ 624,970