Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Revenue from External Customer [Line Items]
Net premiums earned	$ 2,249,087	$ 1,849,956
Insurance
Revenue from External Customer [Line Items]
Net premiums earned		1,604,979
Insurance | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	1,962,835	1,604,979
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	754,456	611,404
Insurance | Short-tail lines
Revenue from External Customer [Line Items]
Net premiums earned	375,212	324,740
Insurance | Workers' compensation
Revenue from External Customer [Line Items]
Net premiums earned	285,423	267,449
Insurance | Commercial automobile
Revenue from External Customer [Line Items]
Net premiums earned	282,234	220,761
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	265,510	180,626
Reinsurance & Monoline Excess
Revenue from External Customer [Line Items]
Net premiums earned		244,977
Reinsurance & Monoline Excess | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	286,252	244,977
Reinsurance & Monoline Excess | Casualty reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	184,122	149,638
Reinsurance & Monoline Excess | Monoline excess
Revenue from External Customer [Line Items]
Net premiums earned	50,234	49,466
Reinsurance & Monoline Excess | Property reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	$ 51,896	$ 45,873