Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) (USD $) In Thousands				9 Months Ended
	Sep. 30, 2011		Dec. 31, 2010	Sep. 30, 2011 Swiss Reinsurance America Corporation		Sep. 30, 2011 Munich Reinsurance America Inc.		Sep. 30, 2011 Everest Reinsurance Company		Sep. 30, 2011 Transatlantic Reinsurance Company		Sep. 30, 2011 National Indemnity Company		Sep. 30, 2011 Partner Reinsurance Europe		Sep. 30, 2011 Scor Holding (Switzerland) AG		Sep. 30, 2011 Berkley Insurance Company		Sep. 30, 2011 General Reinsurance Corporation		Sep. 30, 2011 Lloyd's Syndicate #2003		Sep. 30, 2011 Munchener Ruckversicherungs-Gesellschaft		Sep. 30, 2011 White Mountains Reinsurance of America		Sep. 30, 2011 Platinum Underwriters Re		Sep. 30, 2011 Ace Property and Casualty Insurance Company		Sep. 30, 2011 Allied World Reinsurance		Sep. 30, 2011 AXIS Re Europe		Sep. 30, 2011 Validus Reinsurance Ltd		Sep. 30, 2011 Tower Insurance Company		Sep. 30, 2011 Lloyd Syndicate #4000		Sep. 30, 2011 Lloyd Syndicate #457		Sep. 30, 2011 Insurance Companies		Sep. 30, 2011 All Other
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	$ 170,180		$ 156,869	$ 5,643		$ 12,442		$ 15,019		$ 16,279		$ 14,511		$ 7,082		$ 2,888		$ 1,816		$ 815		$ 5,774		$ 706		$ 150		$ 988		$ 2,013		$ 6,824		$ 5,792		$ 3,130		$ 10,337		$ 2,225		$ 2,806		$ 117,240		$ 52,940
Reinsurance Recoverables Paid/Unpaid Losses	892,778			92,482		75,289		69,873		68,333		26,891		33,691		34,727		30,877		31,741		25,815		27,400		27,530		25,787		22,982		15,656		15,559		14,841		7,112		11,937		10,492		669,015		223,763
Reinsurance Recoverables Total	1,062,958			98,125		87,731		84,892		84,612		41,402		40,773		37,615		32,693		32,556		31,589		28,106		27,680		26,775		24,995		22,480		21,351		17,971		17,449		14,162		13,298		786,255		276,703
Collateral Held	$ 184,170	[1]		$ 7,756	[1]	$ 2,896	[1]	$ 7,193	[1]	$ 6,557	[1]	$ 6,348	[1]	$ 16,832	[1]	$ 7,211	[1]	$ 123	[1]	$ 1,466	[1]	$ 6,401	[1]	$ 5,885	[1]	$ 716	[1]	$ 3,000	[1]	$ 31	[1]	$ 2,431	[1]	$ 5,058	[1]	$ 6,671	[1]	$ 2,577	[1]	$ 1,995	[1]	$ 3,118	[1]	$ 94,265	[1]	$ 89,905	[1]
Rating				A	[2]	A+	[2]	A+	[2]	A	[2]	A++	[2]	AA-	[2]	A	[2]	A+	[2]	A++	[2]	A	[2]	A+	[2]	A	[2]	A	[2]	A+	[2]	A	[2]	A	[2]	A-	[2]	A-	[2]	A	[2]	A	[2]
Rating Agency				AMB	[2]	AMB	[2]	AMB	[2]	AMB	[2]	AMB	[2]	S&P	[2]	AMB	[2]	AMB	[2]	AMB	[2]	AMB	[2]	AMB	[2]	AMB	[2]	AMB	[2]	AMB	[2]	AMB	[2]	AMB	[2]	AMB	[2]	AMB	[2]	AMB	[2]	AMB	[2]

[1] Collateral includes letter of credit balances payable and other balances held by the Company's Insurance Companies and Lloyd's Operations.

[2] A.M. Best Company ("A.M. Best") and Standard and Poor's Rating Services ("S&P").