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 Acquisition Costs	$ 378,026	$ 383,769	$ 362,346
Reserve for Losses and Loss Adjustment Expenses	10,123,215	9,340,183	8,937,858
Unearned Premium Reserves	1,412,778	1,455,219	1,415,402
Premiums Earned	4,101,347	3,934,625	3,894,098
Net Investment Income	620,041	653,463	547,793
Incurred Loss and Loss Adjustment Expenses	3,726,204	2,945,712	2,374,058
Amortization of Deferred Acquisition Costs	950,521	931,855	928,333
Other Operating Expenses	182,403	166,258	167,178
Net Written Premium	4,108,899	3,945,578	3,929,761
International [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs	72,856	79,385	68,561
Reserve for Losses and Loss Adjustment Expenses	2,145,726	1,665,932	1,305,798
Unearned Premium Reserves	263,373	288,721	250,419
Premiums Earned	1,244,492	1,168,130	1,053,538
Net Investment Income	51,232	44,088	37,681
Incurred Loss and Loss Adjustment Expenses	1,372,309	1,050,079	613,251
Amortization of Deferred Acquisition Costs	310,992	288,423	267,121
Other Operating Expenses	27,307	27,646	23,083
Net Written Premium	1,218,561	1,199,594	1,081,337
U.S. [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs	257,155	258,688	249,833
Reserve for Losses and Loss Adjustment Expenses	6,104,930	5,944,708	5,952,679
Unearned Premium Reserves	979,490	998,755	988,901
Premiums Earned	2,133,872	2,028,059	2,055,992
Net Investment Income	263,608	307,274	224,468
Incurred Loss and Loss Adjustment Expenses	1,740,025	1,437,691	1,276,760
Amortization of Deferred Acquisition Costs	465,498	472,344	469,125
Other Operating Expenses	128,791	112,186	119,527
Net Written Premium	2,164,792	2,012,938	2,053,360
Bermuda [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs	48,015	45,696	43,952
Reserve for Losses and Loss Adjustment Expenses	1,872,559	1,729,543	1,679,381
Unearned Premium Reserves	169,915	167,743	176,082
Premiums Earned	722,983	738,436	784,568
Net Investment Income	305,201	302,101	285,644
Incurred Loss and Loss Adjustment Expenses	613,870	457,942	484,047
Amortization of Deferred Acquisition Costs	174,031	171,088	192,087
Other Operating Expenses	26,305	26,426	24,568
Net Written Premium	$ 725,546	$ 733,046	$ 795,064