Industry Segments (Net Premiums Earned By Major Line of Business) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net premiums earned	$ 4,673,516	$ 4,160,867	$ 3,835,582
Specialty Segment [Member]
Net premiums earned	1,646,471	1,442,748	1,288,373
Regional Segment [Member]
Net premiums earned	1,090,217	1,065,975	1,066,922
Alternative Markets Segment [Member]
Net premiums earned	680,334	612,558	608,191
Reinsurance Segment [Member]
Net premiums earned	446,939	426,008	419,356
International Segment [Member]
Net premiums earned	809,555	613,578	452,740
Casualty [Member] | Reinsurance Segment [Member]
Net premiums earned	313,381	307,051	307,474
Property [Member] | Specialty Segment [Member]
Net premiums earned	265,204	237,762	212,164
Property [Member] | Reinsurance Segment [Member]
Net premiums earned	133,558	118,957	111,882
Property [Member] | International Segment [Member]
Net premiums earned	107,798	83,960	47,796
Professional Liability [Member] | Specialty Segment [Member]
Net premiums earned	254,385	229,281	200,219
Professional Liability [Member] | International Segment [Member]
Net premiums earned	103,179	90,871	88,997
Commercial Multiple Peril [Member] | Regional Segment [Member]
Net premiums earned	413,405	394,168	389,997
Commercial Automobile [Member] | Specialty Segment [Member]
Net premiums earned	163,321	141,800	129,505
Commercial Automobile [Member] | Regional Segment [Member]
Net premiums earned	286,332	289,098	301,290
Products Liability [Member] | Specialty Segment [Member]
Net premiums earned	105,584	96,794	112,072
Workers' Compensation [Member] | Regional Segment [Member]
Net premiums earned	227,439	219,639	214,857
Workers' Compensation [Member] | International Segment [Member]
Net premiums earned	82,066	73,823	57,803
Primary Workers Compensation [Member] | Alternative Markets Segment [Member]
Net premiums earned	333,266	271,173	260,508
Excess Workers Compensation [Member] | Alternative Markets Segment [Member]
Net premiums earned	136,576	164,173	216,647
Reinsurance [Member] | International Segment [Member]
Net premiums earned	143,230	82,505	60,907
Accident and Health [Member] | Alternative Markets Segment [Member]
Net premiums earned	136,317	105,128	57,915
Accident and Health [Member] | International Segment [Member]
Net premiums earned	125,927	110,568	99,873
Other Liability [Member] | Specialty Segment [Member]
Net premiums earned	528,623	454,582	384,799
Other Liability [Member] | Alternative Markets Segment [Member]
Net premiums earned	33,609	28,960	24,726
Other Liability [Member] | International Segment [Member]
Net premiums earned	43,829	26,583	20,796
Other [Member] | Specialty Segment [Member]
Net premiums earned	157,594	152,060	146,237
Other [Member] | Regional Segment [Member]
Net premiums earned	163,041	163,070	160,778
Other [Member] | Alternative Markets Segment [Member]
Net premiums earned	40,566	43,124	48,395
Other [Member] | International Segment [Member]
Net premiums earned	$ 27,459	$ 27,447	$ 15,981