Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Revenue from External Customer [Line Items]
Net premiums earned	$ 2,441,961	$ 2,081,018	$ 7,048,205	$ 5,902,598
Insurance
Revenue from External Customer [Line Items]
Net premiums earned		1,819,071		5,151,253
Insurance | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	2,129,014	1,819,071	6,162,005	5,151,253
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	824,595	691,014	2,363,349	1,946,598
Insurance | Short-tail lines
Revenue from External Customer [Line Items]
Net premiums earned	407,211	347,835	1,184,191	1,012,465
Insurance | Workers' compensation
Revenue from External Customer [Line Items]
Net premiums earned	307,323	286,474	892,390	845,394
Insurance | Commercial automobile
Revenue from External Customer [Line Items]
Net premiums earned	307,754	257,314	891,729	715,519
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	282,131	236,434	830,346	631,277
Reinsurance & Monoline Excess
Revenue from External Customer [Line Items]
Net premiums earned		261,947		751,345
Reinsurance & Monoline Excess | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	312,947	261,947	886,200	751,345
Reinsurance & Monoline Excess | Casualty reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	193,726	164,095	568,507	466,264
Reinsurance & Monoline Excess | Monoline excess
Revenue from External Customer [Line Items]
Net premiums earned	54,675	52,361	161,181	146,481
Reinsurance & Monoline Excess | Property reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	$ 64,546	$ 45,491	$ 156,512	$ 138,600