Segment Reporting (Schedule Of Underwriting Results For The Operating Segments) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information [Line Items]
Net written premiums	$ 1,043,104	$ 1,090,790	$ 955,121	$ 1,019,884	$ 942,130	$ 1,085,225	$ 948,970	$ 969,253
Premiums earned	1,005,728	1,044,338	1,039,835	1,011,446	1,020,159	997,265	989,899	927,302	4,101,347	3,934,625	3,894,098
Incurred losses and LAE									3,726,204	2,945,712	2,374,058
Commission and brokerage									950,521	931,855	928,333
Other underwriting expenses									182,403	166,258	167,178
Underwriting gain (loss)									(757,781)	(109,200)	424,529
Premiums earned, percentage		5.00%
U.S. Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross written premiums									1,346,830	1,395,433	1,407,078
Net written premiums									1,344,273	1,392,637	1,397,182
Premiums earned									1,312,713	1,386,951	1,384,873
Incurred losses and LAE									1,034,111	900,938	738,134
Commission and brokerage									327,845	351,559	344,737
Other underwriting expenses									39,290	42,510	44,900
Underwriting gain (loss)									(88,533)	91,944	257,102
Insurance [Member]
Segment Reporting Information [Line Items]
Gross written premiums									975,639	865,371	842,564
Net written premiums									820,519	620,301	656,178
Premiums earned									821,159	641,108	671,119
Incurred losses and LAE									705,914	536,753	538,626
Commission and brokerage									137,653	120,785	124,388
Other underwriting expenses									89,501	69,676	74,627
Underwriting gain (loss)									(111,909)	(86,106)	(66,522)
International [Member]
Segment Reporting Information [Line Items]
Gross written premiums									1,238,444	1,206,953	1,084,476
Net written premiums									1,218,561	1,199,594	1,081,337
Premiums earned									1,244,492	1,168,130	1,053,538
Incurred losses and LAE									1,372,309	1,050,079	613,251
Commission and brokerage									310,992	288,423	267,121
Other underwriting expenses									27,307	27,646	23,083
Underwriting gain (loss)									(466,116)	(198,018)	150,083
Bermuda [Member]
Segment Reporting Information [Line Items]
Gross written premiums									725,261	732,962	794,837
Net written premiums									725,546	733,046	795,064
Premiums earned									722,983	738,436	784,568
Incurred losses and LAE									613,870	457,942	484,047
Commission and brokerage									174,031	171,088	192,087
Other underwriting expenses									26,305	26,426	24,568
Underwriting gain (loss)									$ (91,223)	$ 82,980	$ 83,866
A&H [Member]
Segment Reporting Information [Line Items]
Premiums earned, percentage		2.00%
Other Operating Units [Member]
Segment Reporting Information [Line Items]
Premiums earned, percentage		5.00%