Industry Segments (Identifiable Assets by Segment) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	$ 28,606,913	$ 26,662,144
Revenues	8,098,925	7,902,196	$ 7,691,651
Operating Segments | Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	21,739,360	20,020,455
Revenues	6,478,834	6,397,074	6,208,290
Operating Segments | Insurance | Foreign countries
Segment Reporting, Asset Reconciling Item [Line Items]
Revenues	691,500	725,400	714,200
Operating Segments | Reinsurance & Monoline Excess
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	4,652,074	4,710,445
Revenues	1,009,203	877,551	848,966
Operating Segments | Reinsurance & Monoline Excess | Foreign countries
Segment Reporting, Asset Reconciling Item [Line Items]
Revenues	291,600	249,600	228,100
Corporate, other and eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	2,215,479	1,931,244
Revenues	$ 507,888	$ 506,868	$ 479,907