Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue from External Customer [Line Items]
Net premiums earned	$ 1,581,053	$ 1,568,703	$ 3,148,461	$ 3,138,744
Insurance
Revenue from External Customer [Line Items]
Net premiums earned	1,456,008	1,415,586	2,888,345	2,828,755
Insurance | Other liability
Revenue from External Customer [Line Items]
Net premiums earned	474,721	460,059	938,889	911,889
Insurance | Workers’ compensation
Revenue from External Customer [Line Items]
Net premiums earned	377,346	366,950	743,295	728,086
Insurance | Property
Revenue from External Customer [Line Items]
Net premiums earned	288,398	282,623	583,503	579,413
Insurance | Commercial automobile
Revenue from External Customer [Line Items]
Net premiums earned	175,812	169,599	351,010	340,170
Insurance | Professional liability
Revenue from External Customer [Line Items]
Net premiums earned	139,731	136,355	271,648	269,197
Reinsurance
Revenue from External Customer [Line Items]
Net premiums earned	125,045	153,117	260,116	309,989
Reinsurance | Property
Revenue from External Customer [Line Items]
Net premiums earned	38,204	59,905	82,705	122,037
Reinsurance | Casualty
Revenue from External Customer [Line Items]
Net premiums earned	$ 86,841	$ 93,212	$ 177,411	$ 187,952