Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Revenue from External Customer [Line Items]
Net premiums earned	$ 2,641,914	$ 2,441,961	$ 7,686,073	$ 7,048,205
Insurance
Revenue from External Customer [Line Items]
Net premiums earned		2,129,014		6,162,005
Insurance | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	2,319,435	2,129,014	6,747,704	6,162,005
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	918,054	824,595	2,668,831	2,363,349
Insurance | Short-tail lines
Revenue from External Customer [Line Items]
Net premiums earned	490,598	407,211	1,383,654	1,184,191
Insurance | Commercial automobile
Revenue from External Customer [Line Items]
Net premiums earned	335,572	307,754	965,416	891,729
Insurance | Workers' compensation
Revenue from External Customer [Line Items]
Net premiums earned	300,694	307,323	905,405	892,390
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	274,517	282,131	824,398	830,346
Reinsurance & Monoline Excess
Revenue from External Customer [Line Items]
Net premiums earned		312,947		886,200
Reinsurance & Monoline Excess | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	322,479	312,947	938,369	886,200
Reinsurance & Monoline Excess | Casualty reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	192,071	193,726	574,660	568,507
Reinsurance & Monoline Excess | Monoline excess
Revenue from External Customer [Line Items]
Net premiums earned	60,363	54,675	177,853	161,181
Reinsurance & Monoline Excess | Property reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	$ 70,045	$ 64,546	$ 185,856	$ 156,512