Industry Segments (Net Premiums Earned By Major Line of Business) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Net premiums earned	$ 5,744,418	$ 5,226,537	$ 4,673,516
Insurance-Domestic
Segment Reporting Information [Line Items]
Net premiums earned	4,271,933	3,782,416	3,417,022
Insurance-Domestic | Other Liability [Member]
Segment Reporting Information [Line Items]
Net premiums earned	1,449,425	1,259,376	1,104,369
Insurance-Domestic | Workers' Compensation [Member]
Segment Reporting Information [Line Items]
Net premiums earned	1,126,704	995,047	869,042
Insurance-Domestic | Short-tail Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	875,898	774,809	733,239
Insurance-Domestic | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned	526,344	486,759	455,988
Insurance-Domestic | Professional Liability [Member]
Segment Reporting Information [Line Items]
Net premiums earned	293,562	266,425	254,384
Insurance-International
Segment Reporting Information [Line Items]
Net premiums earned	802,375	723,151	631,841
Insurance-International | Other Liability [Member]
Segment Reporting Information [Line Items]
Net premiums earned	92,411	65,528	44,065
Insurance-International | Workers' Compensation [Member]
Segment Reporting Information [Line Items]
Net premiums earned	71,997	84,637	82,066
Insurance-International | Short-tail Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	415,123	336,814	276,599
Insurance-International | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned	116,369	130,020	125,927
Insurance-International | Professional Liability [Member]
Segment Reporting Information [Line Items]
Net premiums earned	106,475	106,152	103,184
Reinsurance-Global
Segment Reporting Information [Line Items]
Net premiums earned	670,110	720,970	624,653
Reinsurance-Global | Casualty [Member]
Segment Reporting Information [Line Items]
Net premiums earned	487,264	507,790	456,376
Reinsurance-Global | Property [Member]
Segment Reporting Information [Line Items]
Net premiums earned	$ 182,846	$ 213,180	$ 168,277