Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenue from External Customer [Line Items]
Net premiums earned	$ 1,691,418	$ 1,592,856
Insurance
Revenue from External Customer [Line Items]
Net premiums earned		1,427,034
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	547,129	490,661
Insurance | Workers’ compensation
Revenue from External Customer [Line Items]
Net premiums earned	301,600	326,676
Insurance | Property reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	295,478	289,091
Insurance | Commercial automobile
Revenue from External Customer [Line Items]
Net premiums earned	189,643	180,925
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	151,105	139,681
Reinsurance & Monoline Excess
Revenue from External Customer [Line Items]
Net premiums earned		165,822
Reinsurance & Monoline Excess | Property reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	41,571	36,011
Reinsurance & Monoline Excess | Casualty reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	122,731	90,830
Reinsurance & Monoline Excess | Monoline Excess
Revenue from External Customer [Line Items]
Net premiums earned	$ 42,161	$ 38,981