Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) (USD $)				6 Months Ended
	Jun. 30, 2011		Dec. 31, 2010	Jun. 30, 2011 Swiss Reinsurance America Corporation		Jun. 30, 2011 Munich Reinsurance America Inc.		Jun. 30, 2011 Everest Reinsurance Company		Jun. 30, 2011 Transatlantic Reinsurance Company		Jun. 30, 2011 Scor Holding (Switzerland) AG		Jun. 30, 2011 Partner Reinsurance Europe		Jun. 30, 2011 National Indemnity Company		Jun. 30, 2011 Munchener Ruckversicherungs-Gesellschaft		Jun. 30, 2011 Lloyd's Syndicate #2003		Jun. 30, 2011 Berkley Insurance Company		Jun. 30, 2011 General Reinsurance Corporation		Jun. 30, 2011 White Mountains Reinsurance of America		Jun. 30, 2011 Platinum Underwriters Re		Jun. 30, 2011 Ace Property and Casualty Insurance Company		Jun. 30, 2011 AXIS Re Europe		Jun. 30, 2011 Allied World Reinsurance		Jun. 30, 2011 Validus Reinsurance Ltd		Jun. 30, 2011 Tower Insurance Company		Jun. 30, 2011 Lloyd Syndicate #457		Jun. 30, 2011 Lloyd Syndicate #4000		Jun. 30, 2011 Insurance Companies		Jun. 30, 2011 All Other
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	$ 176,960,000		$ 156,869,000	$ 6,800,000		$ 14,100,000		$ 16,300,000		$ 17,800,000		$ 4,200,000		$ 7,500,000		$ 12,700,000		$ 900,000		$ 5,400,000		$ 2,000,000		$ 900,000		$ 200,000		$ 1,300,000		$ 3,200,000		$ 5,700,000		$ 6,900,000		$ 3,000,000		$ 12,600,000		$ 3,900,000		$ 2,600,000		$ 128,000,000		$ 49,000,000
Reinsurance Recoverables Unpaid/Paid Losses	918,900,000			95,700,000		74,900,000		67,500,000		65,900,000		41,600,000		35,500,000		29,600,000		35,100,000		27,000,000		29,800,000		30,600,000		29,600,000		25,400,000		20,800,000		18,100,000		15,600,000		15,700,000		5,600,000		11,100,000		11,400,000		686,500,000		232,400,000
Reinsurance Recoverables Total	1,095,900,000			102,500,000		89,000,000		83,800,000		83,700,000		45,800,000		43,000,000		42,300,000		36,000,000		32,400,000		31,800,000		31,500,000		29,800,000		26,700,000		24,000,000		23,800,000		22,500,000		18,700,000		18,200,000		15,000,000		14,000,000		814,500,000		281,400,000
Collateral Held	$ 197,700,000	[1]		$ 8,200,000	[1]	$ 4,300,000	[1]	$ 7,700,000	[1]	$ 6,800,000	[1]	$ 9,200,000	[1]	$ 18,200,000	[1]	$ 6,100,000	[1]	$ 8,600,000	[1]	$ 6,100,000	[1]			$ 1,300,000	[1]	$ 700,000	[1]	$ 3,100,000	[1]	$ 500,000	[1]	$ 6,100,000	[1]	$ 2,600,000	[1]	$ 6,400,000	[1]	$ 4,400,000	[1]	$ 4,200,000	[1]	$ 2,100,000	[1]	$ 106,600,000	[1]	$ 91,100,000	[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 letters of credit, ceded balances payable and other balances held by the Company's Insurance Companies and Lloyd's Operations.

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