Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Revenue from External Customer [Line Items]
Net premiums earned	$ 3,012,381	$ 2,764,347
Insurance
Revenue from External Customer [Line Items]
Net premiums earned		2,398,768
Insurance | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	2,642,507	2,398,768
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	1,072,728	967,260
Insurance | Short-tail lines
Revenue from External Customer [Line Items]
Net premiums earned	596,109	510,809
Insurance | Auto
Revenue from External Customer [Line Items]
Net premiums earned	389,949	354,013
Insurance | Workers' compensation
Revenue from External Customer [Line Items]
Net premiums earned	311,028	301,495
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	272,693	265,191
Reinsurance and Monoline Excess Segment
Revenue from External Customer [Line Items]
Net premiums earned		365,579
Reinsurance and Monoline Excess Segment | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	369,874	365,579
Reinsurance and Monoline Excess Segment | Casualty
Revenue from External Customer [Line Items]
Net premiums earned	181,767	197,844
Reinsurance and Monoline Excess Segment | Monoline excess
Revenue from External Customer [Line Items]
Net premiums earned	67,264	65,351
Reinsurance and Monoline Excess Segment | Property
Revenue from External Customer [Line Items]
Net premiums earned	$ 120,843	$ 102,384