Industry Segments (Net Premiums Earned By Major Line of Business) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Net premiums earned	$ 1,186,533	$ 1,055,823	$ 3,433,541	$ 3,055,564
Specialty Segment [Member]
Net premiums earned	420,165	367,417	1,215,417	1,047,567
Regional Segment [Member]
Net premiums earned	276,629	267,142	809,072	795,423
Alternative Markets Segment [Member]
Net premiums earned	177,041	156,820	506,149	454,156
Reinsurance Segment [Member]
Net premiums earned	111,599	103,906	327,452	315,220
International Segment [Member]
Net premiums earned	201,099	160,538	575,451	443,198
Premises Operations [Member] | Specialty Segment [Member]
Net premiums earned	135,662	119,496	386,491	331,457
Property [Member] | Specialty Segment [Member]
Net premiums earned	68,767	59,578	202,602	174,690
Property [Member] | Reinsurance Segment [Member]
Net premiums earned	30,956	27,989	94,754	86,691
Property [Member] | International Segment [Member]
Net premiums earned	27,418	22,327	79,039	59,430
Professional Liability [Member] | Specialty Segment [Member]
Net premiums earned	63,519	58,739	188,680	167,748
Professional Liability [Member] | International Segment [Member]
Net premiums earned	24,348	24,982	74,356	69,875
Commercial Automobile [Member] | Specialty Segment [Member]
Net premiums earned	42,335	36,440	121,027	102,055
Commercial Automobile [Member] | Regional Segment [Member]
Net premiums earned	72,486	72,537	213,398	217,021
Products Liability [Member] | Specialty Segment [Member]
Net premiums earned	27,444	24,954	78,731	71,856
Other [Member] | Specialty Segment [Member]
Net premiums earned	38,990	39,044	115,658	113,078
Other [Member] | Regional Segment [Member]
Net premiums earned	42,618	41,272	122,887	122,770
Other [Member] | Alternative Markets Segment [Member]
Net premiums earned	11,269	10,304	31,147	31,371
Commercial Multiple Peril [Member] | Regional Segment [Member]
Net premiums earned	104,331	98,200	305,978	292,030
Workers' Compensation [Member] | Specialty Segment [Member]
Net premiums earned	43,448	29,166	122,228	86,683
Workers' Compensation [Member] | Regional Segment [Member]
Net premiums earned	57,194	55,133	166,809	163,602
Workers' Compensation [Member] | International Segment [Member]
Net premiums earned	21,102	18,884	59,618	54,480
Marine [Domain] | International Segment [Member]
Net premiums earned	21,741	14,487	59,125	36,050
Primary Workers Compensation [Member] | Alternative Markets Segment [Member]
Net premiums earned	88,655	70,172	243,494	199,768
Casualty [Member] | Reinsurance Segment [Member]
Net premiums earned	80,643	75,917	232,698	228,529
Excess Workers Compensation [Member] | Alternative Markets Segment [Member]
Net premiums earned	34,179	40,038	105,051	126,940
Accident and Health | Alternative Markets Segment [Member]
Net premiums earned	34,461	27,894	101,701	75,002
Accident and Health | International Segment [Member]
Net premiums earned	11,870	9,994	39,932	30,515
Property Reinsurance [Domain] | International Segment [Member]
Net premiums earned	11,250	6,680	31,281	16,062
Automobile [Member] | International Segment [Member]
Net premiums earned	30,557	26,334	90,831	80,585
Casualty Reinsurance [Domain] | International Segment [Member]
Net premiums earned	32,378	19,650	88,686	52,717
Other Liability [Member] | Alternative Markets Segment [Member]
Net premiums earned	8,477	8,412	24,756	21,075
Other Liability [Member] | International Segment [Member]
Net premiums earned	14,308	9,348	31,919	23,780
Fidelity and Surety [Domain] | International Segment [Member]
Net premiums earned	$ 6,127	$ 7,852	$ 20,664	$ 19,704