Industry Segments (Net Premiums Earned By Major Line of Business) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Net premiums earned	$ 5,226,537	$ 4,673,516	$ 4,160,867
Insurance-Domestic
Segment Reporting Information [Line Items]
Net premiums earned	3,782,416	3,417,022	3,121,281
Insurance-Domestic | Other Liability [Member]
Segment Reporting Information [Line Items]
Net premiums earned	1,259,376	1,104,369	1,003,080
Insurance-Domestic | Workers' Compensation [Member]
Segment Reporting Information [Line Items]
Net premiums earned	995,047	869,042	785,454
Insurance-Domestic | Short-tail Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	774,809	733,239	665,575
Insurance-Domestic | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned	486,759	455,988	437,891
Insurance-Domestic | Professional Liability [Member]
Segment Reporting Information [Line Items]
Net premiums earned	266,425	254,384	229,281
Insurance-International
Segment Reporting Information [Line Items]
Net premiums earned	723,151	631,841	508,509
Insurance-International | Other Liability [Member]
Segment Reporting Information [Line Items]
Net premiums earned	65,528	44,065	49,800
Insurance-International | Workers' Compensation [Member]
Segment Reporting Information [Line Items]
Net premiums earned	84,637	82,066	73,823
Insurance-International | Short-tail Lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned	336,814	276,599	183,447
Insurance-International | Commercial Automobile [Member]
Segment Reporting Information [Line Items]
Net premiums earned	130,020	125,927	110,568
Insurance-International | Professional Liability [Member]
Segment Reporting Information [Line Items]
Net premiums earned	106,152	103,184	90,871
Reinsurance-Global
Segment Reporting Information [Line Items]
Net premiums earned	720,970	624,653	531,077
Reinsurance-Global | Casualty [Member]
Segment Reporting Information [Line Items]
Net premiums earned	507,790	456,376	389,557
Reinsurance-Global | Property [Member]
Segment Reporting Information [Line Items]
Net premiums earned	$ 213,180	$ 168,277	$ 141,520