SEGMENT INFORMATION (DETAILS 3) (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
Revenue by Product [Line Items]
Net premiums earned	$ 846,753	$ 839,992	$ 840,014	$ 788,201	$ 757,318	$ 758,873	$ 735,027	$ 696,192	$ 3,314,961	$ 2,947,410	$ 2,791,764
Insurance [Member]
Revenue by Product [Line Items]
Net premiums earned									1,429,687	1,206,493	1,157,966
Insurance [Member] | Property [Member]
Revenue by Product [Line Items]
Net premiums earned									385,291	337,525	268,469
Insurance [Member] | Marine [Member]
Revenue by Product [Line Items]
Net premiums earned									152,123	145,356	139,196
Insurance [Member] | Terrorism [Member]
Revenue by Product [Line Items]
Net premiums earned									35,213	32,486	34,001
Insurance [Member] | Aviation [Member]
Revenue by Product [Line Items]
Net premiums earned									70,681	66,636	64,245
Insurance [Member] | Credit And Political Risk [Member]
Revenue by Product [Line Items]
Net premiums earned									97,680	89,773	188,311
Insurance [Member] | Professional Lines [Member]
Revenue by Product [Line Items]
Net premiums earned									536,238	444,663	381,364
Insurance [Member] | Liability [Member]
Revenue by Product [Line Items]
Net premiums earned									89,555	87,481	82,380
Insurance [Member] | Accident And Health [Member]
Revenue by Product [Line Items]
Net premiums earned									62,906	2,573	0
Reinsurance [Member]
Revenue by Product [Line Items]
Net premiums earned									1,885,274	1,740,917	1,633,798
Reinsurance [Member] | Property [Member]
Revenue by Product [Line Items]
Net premiums earned									356,022	323,201	311,272
Reinsurance [Member] | Professional Lines [Member]
Revenue by Product [Line Items]
Net premiums earned									281,025	285,224	266,792
Reinsurance [Member] | Liability [Member]
Revenue by Product [Line Items]
Net premiums earned									230,872	232,014	227,511
Reinsurance [Member] | Other [Member]
Revenue by Product [Line Items]
Net premiums earned									28,028	29,082	31,851
Reinsurance [Member] | Catastrophe [Member]
Revenue by Product [Line Items]
Net premiums earned									456,858	454,954	451,085
Reinsurance [Member] | Credit And Bond [Member]
Revenue by Product [Line Items]
Net premiums earned									263,912	217,809	179,362
Reinsurance [Member] | Motor [Member]
Revenue by Product [Line Items]
Net premiums earned									202,830	127,404	99,497
Reinsurance [Member] | Engineering [Member]
Revenue by Product [Line Items]
Net premiums earned									$ 65,727	$ 71,229	$ 66,428