Industry Segments (Net Premiums Earned By Major Line of Business) (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Net premiums earned	$ 1,055,823	$ 967,297	$ 3,055,564	$ 2,845,986
Specialty Segment [Member] | Premises Operations [Member]
Net premiums earned	119,496	98,699	331,457	286,458
Reinsurance Segment [Member] | Casualty [Member]
Net premiums earned	75,917	82,273	228,529	230,418
Specialty Segment [Member] | Property [Member]
Net premiums earned	59,578	56,525	174,690	156,447
Reinsurance Segment [Member] | Property [Member]
Net premiums earned	27,989	20,853	86,691	77,898
International Segment [Member] | Property [Member]
Net premiums earned	38,452	24,189	106,038	53,001
Specialty Segment [Member] | Professional Liability [Member]
Net premiums earned	58,739	52,092	167,748	147,129
International Segment [Member] | Professional Liability [Member]
Net premiums earned	24,982	20,207	69,875	63,978
Regional Segment [Member] | Commercial Multiple Peril [Member]
Net premiums earned	98,200	97,642	292,030	290,755
Regional Segment [Member] | Commercial Automobile [Member]
Net premiums earned	72,537	75,082	217,021	226,719
Specialty Segment [Member] | Commercial Automobile [Member]
Net premiums earned	36,440	30,056	102,055	98,684
Specialty Segment [Member] | Products Liability [Member]
Net premiums earned	24,954	23,870	71,856	87,106
Regional Segment [Member] | Workers' Compensation [Member]
Net premiums earned	55,133	53,788	163,602	160,466
International Segment [Member] | Workers' Compensation [Member]
Net premiums earned	18,884	14,555	54,480	41,606
Alternative Markets Segment [Member] | Primary Workers Compensation [Member]
Net premiums earned	70,172	64,421	199,768	192,393
Alternative Markets Segment [Member] | Excess Workers Compensation [Member]
Net premiums earned	40,038	53,485	126,940	169,536
International Segment [Member] | Reinsurance [Member]
Net premiums earned	26,330	20,791	68,779	51,691
International Segment [Member] | Automobile [Member]
Net premiums earned	18,428	18,313	53,425	52,996
International Segment [Member] | Other Liability [Member]
Net premiums earned	16,323	10,822	41,785	30,868
Regional Segment [Member] | Other [Member]
Net premiums earned	41,272	41,577	122,770	120,447
Specialty Segment [Member] | Other [Member]
Net premiums earned	68,210	64,997	199,761	179,881
Alternative Markets Segment [Member] | Other [Member]
Net premiums earned	46,610	30,924	127,448	96,913
International Segment [Member] | Other [Member]
Net premiums earned	17,139	12,136	48,816	30,596
Specialty Segment [Member]
Net premiums earned	367,417	326,239	1,047,567	955,705
Regional Segment [Member]
Net premiums earned	267,142	268,089	795,423	798,387
Alternative Markets Segment [Member]
Net premiums earned	156,820	148,830	454,156	458,842
Reinsurance Segment [Member]
Net premiums earned	103,906	103,126	315,220	308,316
International Segment [Member]
Net premiums earned	$ 160,538	$ 121,013	$ 443,198	$ 324,736