Industry Segments (Net Premiums Earned By Major Line of Business) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2012
Net premiums earned	$ 1,147,357	$ 1,017,244	$ 1,999,741	$ 2,247,008
Specialty Segment [Member]
Net premiums earned	408,142	349,943	680,150	795,252
Regional Segment [Member]
Net premiums earned	268,177	266,764	528,281	532,443
Alternative Markets Segment [Member]
Net premiums earned	170,415	148,999	297,336	329,108
Reinsurance Segment [Member]
Net premiums earned	109,515	105,836	211,314	215,853
International Segment [Member]
Net premiums earned	191,108	145,702	282,660	374,352
Premises Operations [Member] | Specialty Segment [Member]
Net premiums earned	128,590	110,651	211,961	250,829
Property [Member] | Specialty Segment [Member]
Net premiums earned	70,818	60,718	115,112	133,835
Property [Member] | Reinsurance Segment [Member]
Net premiums earned	34,337	29,910	58,702	63,798
Property [Member] | International Segment [Member]
Net premiums earned	26,727	20,568	37,103	51,621
Professional Liability [Member] | Specialty Segment [Member]
Net premiums earned	62,469	55,497	109,009	125,161
Professional Liability [Member] | International Segment [Member]
Net premiums earned	23,421	21,591	44,893	50,008
Commercial Automobile [Member] | Specialty Segment [Member]
Net premiums earned	40,359	34,168	65,615	78,692
Commercial Automobile [Member] | Regional Segment [Member]
Net premiums earned	70,501	72,355	144,484	140,912
Products Liability [Member] | Specialty Segment [Member]
Net premiums earned	25,854	23,500	46,902	51,287
Other [Member] | Specialty Segment [Member]
Net premiums earned	38,137	35,813	74,034	76,668
Other [Member] | Regional Segment [Member]
Net premiums earned	40,512	42,384	81,498	80,269
Other [Member] | Alternative Markets Segment [Member]
Net premiums earned	10,023	11,203	21,067	19,878
Commercial Multiple Peril [Member] | Regional Segment [Member]
Net premiums earned	101,282	97,123	193,830	201,647
Workers' Compensation [Member] | Specialty Segment [Member]
Net premiums earned	41,915	29,596	57,517	78,780
Workers' Compensation [Member] | Regional Segment [Member]
Net premiums earned	55,882	54,902	108,469	109,615
Workers' Compensation [Member] | International Segment [Member]
Net premiums earned	19,898	18,258	35,596	38,516
Marine [Domain] | International Segment [Member]
Net premiums earned	21,225	11,895	21,563	37,384
Primary Workers Compensation [Member] | Alternative Markets Segment [Member]
Net premiums earned	79,754	65,421	129,596	154,839
Casualty [Member] | Reinsurance Segment [Member]
Net premiums earned	75,178	75,926	152,612	152,055
Excess Workers Compensation [Member] | Alternative Markets Segment [Member]
Net premiums earned	35,279	43,169	86,902	70,872
Accident and Health | Alternative Markets Segment [Member]
Net premiums earned	36,593	23,060	47,108	67,240
Accident and Health | International Segment [Member]
Net premiums earned	16,235	10,857	20,521	28,062
Property Reinsurance [Domain] | International Segment [Member]
Net premiums earned	8,721	5,328	9,382	20,031
Automobile [Member] | International Segment [Member]
Net premiums earned	29,606	26,097	54,251	60,274
Casualty Reinsurance [Domain] | International Segment [Member]
Net premiums earned	28,095	16,294	33,067	56,308
Other Liability [Member] | Alternative Markets Segment [Member]
Net premiums earned	8,766	6,146	12,663	16,279
Other Liability [Member] | International Segment [Member]
Net premiums earned	$ 9,857	$ 7,799	$ 14,432	$ 17,611