Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenue from External Customer [Line Items]
Net premiums earned	$ 1,581,500	$ 1,585,944	$ 4,720,244	$ 4,673,072
Insurance
Revenue from External Customer [Line Items]
Net premiums earned	1,433,729	1,423,635	4,262,485	4,180,985
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	466,616	455,841	1,378,505	1,302,841
Insurance | Workers’ compensation
Revenue from External Customer [Line Items]
Net premiums earned	378,529	354,185	1,106,616	1,042,503
Insurance | Property
Revenue from External Customer [Line Items]
Net premiums earned	287,860	312,865	887,791	962,435
Insurance | Commercial automobile
Revenue from External Customer [Line Items]
Net premiums earned	163,277	164,540	482,929	481,249
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	137,447	136,204	406,644	391,957
Reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	147,771	162,309	457,759	492,087
Reinsurance | Property
Revenue from External Customer [Line Items]
Net premiums earned	53,293	65,156	175,329	190,516
Reinsurance | Casualty
Revenue from External Customer [Line Items]
Net premiums earned	$ 94,478	$ 97,153	$ 282,430	$ 301,571