Industry Segments (Identifiable Assets by Segment) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	$ 32,086,414	$ 28,606,913
Revenues	9,455,466	8,098,925	$ 7,902,196
Operating Segments | Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	24,414,305	21,739,360
Revenues	7,578,592	6,478,834	6,397,074
Operating Segments | Insurance | Foreign countries
Segment Reporting, Asset Reconciling Item [Line Items]
Revenues	873,000	692,000	725,000
Operating Segments | Reinsurance & Monoline Excess
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	4,916,894	4,652,074
Revenues	1,203,647	1,009,203	877,551
Operating Segments | Reinsurance & Monoline Excess | Foreign countries
Segment Reporting, Asset Reconciling Item [Line Items]
Revenues	380,000	292,000	250,000
Corporate, other and eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	2,755,215	2,215,479
Revenues	$ 582,595	$ 507,888	$ 506,868