Industry Segments (Net Premiums Earned By Major Line of Business) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Net premiums earned	$ 4,160,867	$ 3,835,582	$ 3,805,849
Specialty Segment [Member]
Net premiums earned	1,442,748	1,288,373	1,354,355
Regional Segment [Member]
Net premiums earned	1,065,975	1,066,922	1,116,871
Alternative Markets Segment [Member]
Net premiums earned	612,558	608,191	597,932
Reinsurance Segment [Member]
Net premiums earned	426,008	419,356	411,511
International Segment [Member]
Net premiums earned	613,578	452,740	325,180
Casualty [Member] | Reinsurance Segment [Member]
Net premiums earned	307,051	307,474	323,479
Property [Member] | Specialty Segment [Member]
Net premiums earned	237,762	212,164	199,746
Property [Member] | Reinsurance Segment [Member]
Net premiums earned	118,957	111,882	88,032
Property [Member] | International Segment [Member]
Net premiums earned	151,177	80,105	26,119
Professional Liability [Member] | Specialty Segment [Member]
Net premiums earned	229,281	200,219	173,201
Professional Liability [Member] | International Segment [Member]
Net premiums earned	90,871	88,997	84,101
Commercial Multiple Peril [Member] | Regional Segment [Member]
Net premiums earned	394,168	389,997	405,552
Commercial Automobile [Member] | Specialty Segment [Member]
Net premiums earned	141,800	129,505	189,501
Commercial Automobile [Member] | Regional Segment [Member]
Net premiums earned	289,098	301,290	322,445
Products Liability [Member] | Specialty Segment [Member]
Net premiums earned	96,794	112,072	131,713
Workers' Compensation [Member] | Regional Segment [Member]
Net premiums earned	219,639	214,857	229,066
Workers' Compensation [Member] | International Segment [Member]
Net premiums earned	73,823	57,802	46,698
Primary Workers Compensation [Member] | Alternative Markets Segment [Member]
Net premiums earned	271,173	260,508	242,259
Excess Workers Compensation [Member] | Alternative Markets Segment [Member]
Net premiums earned	164,173	216,647	252,196
Reinsurance [Member] | International Segment [Member]
Net premiums earned	104,851	74,047	56,454
Accident and Health [Member] | Alternative Markets Segment [Member]
Net premiums earned	105,128	57,915	36,414
Accident and Health [Member] | International Segment [Member]
Net premiums earned	72,723	69,875	64,969
Other Liability [Member] | Specialty Segment [Member]
Net premiums earned	454,582	384,799	449,120
Other Liability [Member] | Alternative Markets Segment [Member]
Net premiums earned	28,960	24,726	23,412
Other Liability [Member] | International Segment [Member]
Net premiums earned	52,189	38,344	25,736
Other [Member] | Specialty Segment [Member]
Net premiums earned	282,529	249,614	211,074
Other [Member] | Regional Segment [Member]
Net premiums earned	163,070	160,778	159,808
Other [Member] | Alternative Markets Segment [Member]
Net premiums earned	43,124	48,395	43,651
Other [Member] | International Segment [Member]
Net premiums earned	$ 67,944	$ 43,570	$ 21,103