Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Revenue from External Customer [Line Items]
Net premiums earned	$ 2,491,432	$ 2,249,087
Insurance
Revenue from External Customer [Line Items]
Net premiums earned		1,962,835
Insurance | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	2,181,876	1,962,835
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	857,149	749,391
Insurance | Short-tail lines
Revenue from External Customer [Line Items]
Net premiums earned	430,503	375,212
Insurance | Commercial automobile
Revenue from External Customer [Line Items]
Net premiums earned	311,422	282,234
Insurance | Workers' compensation
Revenue from External Customer [Line Items]
Net premiums earned	305,561	285,423
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	277,241	270,575
Reinsurance & Monoline Excess
Revenue from External Customer [Line Items]
Net premiums earned		286,252
Reinsurance & Monoline Excess | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	309,556	286,252
Reinsurance & Monoline Excess | Casualty reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	193,730	184,122
Reinsurance & Monoline Excess | Monoline excess
Revenue from External Customer [Line Items]
Net premiums earned	58,644	51,896
Reinsurance & Monoline Excess | Property reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	$ 57,182	$ 50,234