Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenue from External Customer [Line Items]
Net premiums earned	$ 1,849,956	$ 1,691,418
Insurance
Revenue from External Customer [Line Items]
Net premiums earned		1,484,955
Insurance | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	1,604,979	1,484,955
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	601,902	547,129
Insurance | Short-tail lines
Revenue from External Customer [Line Items]
Net premiums earned	324,740	295,478
Insurance | Workers' compensation
Revenue from External Customer [Line Items]
Net premiums earned	267,449	301,600
Insurance | Commercial automobile
Revenue from External Customer [Line Items]
Net premiums earned	220,761	189,643
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	190,128	151,105
Reinsurance & Monoline Excess
Revenue from External Customer [Line Items]
Net premiums earned		206,463
Reinsurance & Monoline Excess | Operating Segments
Revenue from External Customer [Line Items]
Net premiums earned	244,977	206,463
Reinsurance & Monoline Excess | Casualty reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	149,638	122,731
Reinsurance & Monoline Excess | Monoline excess
Revenue from External Customer [Line Items]
Net premiums earned	45,873	42,161
Reinsurance & Monoline Excess | Property reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	$ 49,466	$ 41,571