Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue from External Customer [Line Items]
Net premiums earned	$ 1,603,485	$ 1,581,500	$ 4,751,947	$ 4,720,244
Insurance
Revenue from External Customer [Line Items]
Net premiums earned	1,488,658	1,433,729	4,377,003	4,262,485
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	482,255	466,616	1,421,144	1,378,505
Insurance | Workers’ compensation
Revenue from External Customer [Line Items]
Net premiums earned	378,922	378,529	1,122,217	1,106,616
Insurance | Property
Revenue from External Customer [Line Items]
Net premiums earned	299,856	276,737	883,359	856,150
Insurance | Commercial automobile
Revenue from External Customer [Line Items]
Net premiums earned	185,069	174,400	536,079	514,570
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	142,556	137,447	414,204	406,644
Reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	114,827	147,771	374,944	457,759
Reinsurance | Property
Revenue from External Customer [Line Items]
Net premiums earned	27,788	53,293	110,494	175,329
Reinsurance | Casualty
Revenue from External Customer [Line Items]
Net premiums earned	$ 87,039	$ 94,478	$ 264,450	$ 282,430