Schedule III - Supplementary Insurance Information (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Cost	$ 448,795	$ 405,942	$ 391,360
Reserve for Losses and Loss Expenses	9,337,134	9,016,549	9,071,671
Unearned Premiums	2,189,575	1,953,721	1,928,428
Premiums Earned	4,160,867	3,835,582	3,805,849
Net Investment Income	526,351	530,525	379,008
Loss and Loss Expenses	2,658,365	2,309,867	2,336,707
Amortization of Deferred Policy Acquisition Cost	1,034,400	917,217	903,154
Other Operating Cost and Expenses	586,929	786,711	721,098
Net Premiums Written	4,357,368	3,850,926	3,730,095
Specialty Segment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Cost	147,245	122,440	116,234
Reserve for Losses and Loss Expenses	3,306,535	3,186,769	3,210,479
Unearned Premiums	822,186	693,358	662,972
Premiums Earned	1,442,748	1,288,373	1,354,355
Net Investment Income	175,289	180,063	125,351
Loss and Loss Expenses	857,223	750,831	838,894
Amortization of Deferred Policy Acquisition Cost	312,230	263,279	297,388
Other Operating Cost and Expenses	158,529	160,811	126,078
Net Premiums Written	1,554,516	1,311,831	1,260,451
Regional Segment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Cost	139,519	139,289	142,249
Reserve for Losses and Loss Expenses	1,370,748	1,378,936	1,440,158
Unearned Premiums	530,171	530,672	554,426
Premiums Earned	1,065,975	1,066,922	1,116,871
Net Investment Income	75,404	82,411	57,530
Loss and Loss Expenses	725,195	647,986	686,093
Amortization of Deferred Policy Acquisition Cost	302,149	293,690	289,973
Other Operating Cost and Expenses	85,764	93,418	94,982
Net Premiums Written	1,064,507	1,044,347	1,081,100
Alternative Markets Segment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Cost	34,928	33,471	34,443
Reserve for Losses and Loss Expenses	2,506,424	2,355,007	2,221,488
Unearned Premiums	287,246	261,858	287,031
Premiums Earned	612,558	608,191	597,932
Net Investment Income	121,360	123,309	83,719
Loss and Loss Expenses	442,721	410,873	378,961
Amortization of Deferred Policy Acquisition Cost	99,938	92,364	89,432
Other Operating Cost and Expenses	131,260	128,829	137,415
Net Premiums Written	619,097	582,045	589,637
Reinsurance Segment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Cost	58,143	56,834	60,219
Reserve for Losses and Loss Expenses	1,523,210	1,591,397	1,787,006
Unearned Premiums	207,956	203,430	225,209
Premiums Earned	426,008	419,356	411,511
Net Investment Income	91,871	103,079	75,505
Loss and Loss Expenses	262,286	220,230	238,075
Amortization of Deferred Policy Acquisition Cost	130,354	129,508	127,446
Other Operating Cost and Expenses	41,989	42,775	35,137
Net Premiums Written	430,329	401,239	423,425
International Segment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Cost	68,960	53,908	38,215
Reserve for Losses and Loss Expenses	630,217	504,440	412,540
Unearned Premiums	342,016	264,403	198,790
Premiums Earned	613,578	452,740	325,180
Net Investment Income	42,882	32,794	26,767
Loss and Loss Expenses	370,940	279,947	194,684
Amortization of Deferred Policy Acquisition Cost	189,729	138,376	98,915
Other Operating Cost and Expenses	55,707	46,037	35,629
Net Premiums Written	688,919	511,464	375,482
Corporate, Other and Eliminations Segment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Cost	0	0	0
Reserve for Losses and Loss Expenses	0	0	0
Unearned Premiums	0	0	0
Premiums Earned	0	0	0
Net Investment Income	19,545	8,869	10,136
Loss and Loss Expenses	0	0	0
Amortization of Deferred Policy Acquisition Cost	0	0	0
Other Operating Cost and Expenses	113,680	314,841	291,857
Net Premiums Written	$ 0	$ 0	$ 0