Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenue from External Customer [Line Items]
Premiums Earned, Net	$ 1,568,703	$ 1,559,793	$ 3,138,744	$ 3,087,128
Insurance
Revenue from External Customer [Line Items]
Premiums Earned, Net	1,415,586	1,390,744	2,828,755	2,757,350
Insurance | Other liability
Revenue from External Customer [Line Items]
Premiums Earned, Net	460,059	433,472	911,889	847,000
Insurance | Workers’ compensation
Revenue from External Customer [Line Items]
Premiums Earned, Net	366,950	344,736	728,086	688,317
Insurance | Property
Revenue from External Customer [Line Items]
Premiums Earned, Net	290,696	322,493	599,930	649,570
Insurance | Commercial automobile
Revenue from External Customer [Line Items]
Premiums Earned, Net	161,526	159,892	319,653	316,709
Insurance | Professional liability
Revenue from External Customer [Line Items]
Premiums Earned, Net	136,355	130,151	269,197	255,754
Reinsurance
Revenue from External Customer [Line Items]
Premiums Earned, Net	153,117	169,049	309,989	329,778
Reinsurance | Property
Revenue from External Customer [Line Items]
Premiums Earned, Net	59,905	68,936	122,037	125,360
Reinsurance | Casualty
Revenue from External Customer [Line Items]
Premiums Earned, Net	$ 93,212	$ 100,113	$ 187,952	$ 204,418