Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue from External Customer [Line Items]
Net premiums earned	$ 1,567,408	$ 1,570,042
Insurance
Revenue from External Customer [Line Items]
Net premiums earned	1,432,337	1,413,170
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	464,167	451,830
Insurance | Workers’ compensation
Revenue from External Customer [Line Items]
Net premiums earned	365,948	361,136
Insurance | Property
Revenue from External Customer [Line Items]
Net premiums earned	295,105	296,790
Insurance | Commercial automobile
Revenue from External Customer [Line Items]
Net premiums earned	175,199	170,571
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	131,918	132,843
Reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	135,071	156,872
Reinsurance | Property
Revenue from External Customer [Line Items]
Net premiums earned	44,501	62,133
Reinsurance | Casualty
Revenue from External Customer [Line Items]
Net premiums earned	$ 90,570	$ 94,739