Industry Segments (Identifiable Assets by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	$ 26,643,428				$ 24,895,977				$ 26,643,428	$ 24,895,977
Revenues	1,976,074	$ 1,965,716	$ 2,023,384	$ 1,937,022	1,951,586	$ 1,937,902	$ 1,910,916	$ 1,891,247	7,902,196	7,691,651	$ 7,684,764
Operating Segments | Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	20,003,202				18,214,293				20,003,202	18,214,293
Revenues									6,397,074	6,208,290	6,003,130
Operating Segments | Insurance | Foreign countries
Segment Reporting, Asset Reconciling Item [Line Items]
Revenues									725,400	714,200	688,200
Operating Segments | Reinsurance & Monoline Excess
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	4,709,724				4,371,151				4,709,724	4,371,151
Revenues									877,551	848,966	922,478
Operating Segments | Reinsurance & Monoline Excess | Foreign countries
Segment Reporting, Asset Reconciling Item [Line Items]
Revenues									249,600	228,100	201,300
Corporate, other and eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Identifiable Assets	$ 1,930,502				$ 2,310,533				1,930,502	2,310,533
Revenues									$ 506,868	$ 479,907	$ 423,298