Industry Segments (Net Premiums Earned By Major Line of Business) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Net premiums earned	$ 1,099,651	$ 982,497
Specialty Segment [Member]
Net premiums earned	387,110	330,207
Regional Segment [Member]
Net premiums earned	264,266	261,517
Alternative Markets Segment [Member]
Net premiums earned	158,693	148,337
Reinsurance Segment [Member]
Net premiums earned	106,338	105,478
International Segment [Member]
Net premiums earned	183,244	136,958
Premises Operations [Member] | Specialty Segment [Member]
Net premiums earned	122,239	101,310
Property [Member] | Specialty Segment [Member]
Net premiums earned	63,017	54,394
Property [Member] | Reinsurance Segment [Member]
Net premiums earned	29,461	28,792
Property [Member] | International Segment [Member]
Net premiums earned	37,590	31,643
Professional Liability [Member] | Specialty Segment [Member]
Net premiums earned	62,692	53,512
Professional Liability [Member] | International Segment [Member]
Net premiums earned	28,624	23,302
Commercial Automobile [Member] | Specialty Segment [Member]
Net premiums earned	38,333	31,447
Commercial Automobile [Member] | Regional Segment [Member]
Net premiums earned	70,411	72,129
Products Liability [Member] | Specialty Segment [Member]
Net premiums earned	25,433	23,402
Other [Member] | Specialty Segment [Member]
Net premiums earned	75,396	66,142
Other [Member] | Regional Segment [Member]
Net premiums earned	39,757	39,114
Other [Member] | Alternative Markets Segment [Member]
Net premiums earned	9,855	9,864
Other [Member] | International Segment [Member]
Net premiums earned	28,742	14,573
Commercial Multiple Peril [Member] | Regional Segment [Member]
Net premiums earned	100,365	96,707
Workers' Compensation [Member] | Regional Segment [Member]
Net premiums earned	53,733	53,567
Workers' Compensation [Member] | International Segment [Member]
Net premiums earned	18,632	17,338
Primary Workers Compensation [Member] | Alternative Markets Segment [Member]
Net premiums earned	75,085	64,175
Casualty [Member] | Reinsurance Segment [Member]
Net premiums earned	76,877	76,686
Excess Workers Compensation [Member] | Alternative Markets Segment [Member]
Net premiums earned	35,593	43,733
Accident and Health | Alternative Markets Segment [Member]
Net premiums earned	30,647	24,048
Reinsurance [Member] | International Segment [Member]
Net premiums earned	17,330	20,827
Automobile [Member] | International Segment [Member]
Net premiums earned	30,735	17,969
Other Liability [Member] | Alternative Markets Segment [Member]
Net premiums earned	7,513	6,517
Other Liability [Member] | International Segment [Member]
Net premiums earned	$ 21,591	$ 11,306