Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) (USD $) In Thousands, unless otherwise specified						9 Months Ended				9 Months Ended				9 Months Ended				9 Months Ended				9 Months Ended				9 Months Ended				9 Months Ended				9 Months Ended				9 Months Ended				9 Months Ended				9 Months Ended				9 Months Ended			9 Months Ended				9 Months Ended				9 Months Ended				9 Months Ended				9 Months Ended				9 Months Ended				9 Months Ended				9 Months Ended
	Sep. 30, 2012		Dec. 31, 2011	Sep. 30, 2012 Swiss Reinsurance America Corporation		Sep. 30, 2012 Swiss Reinsurance America Corporation AMB	Sep. 30, 2012 Swiss Reinsurance America Corporation S&P	Sep. 30, 2012 Munich Reinsurance America Inc.		Sep. 30, 2012 Munich Reinsurance America Inc. AMB	Sep. 30, 2012 Munich Reinsurance America Inc. S&P	Sep. 30, 2012 Transatlantic Reinsurance Company		Sep. 30, 2012 Transatlantic Reinsurance Company AMB	Sep. 30, 2012 Transatlantic Reinsurance Company S&P	Sep. 30, 2012 Everest Reinsurance Company		Sep. 30, 2012 Everest Reinsurance Company AMB	Sep. 30, 2012 Everest Reinsurance Company S&P	Sep. 30, 2012 National Indemnity Company		Sep. 30, 2012 National Indemnity Company AMB	Sep. 30, 2012 National Indemnity Company S&P	Sep. 30, 2012 Partner Reinsurance Europe		Sep. 30, 2012 Partner Reinsurance Europe AMB	Sep. 30, 2012 Partner Reinsurance Europe S&P	Sep. 30, 2012 Lloyd Syndicate #2003		Sep. 30, 2012 Lloyd Syndicate #2003 AMB	Sep. 30, 2012 Lloyd Syndicate #2003 S&P	Sep. 30, 2012 Allied World Reinsurance		Sep. 30, 2012 Allied World Reinsurance AMB	Sep. 30, 2012 Allied World Reinsurance S&P	Sep. 30, 2012 Berkley Insurance Company		Sep. 30, 2012 Berkley Insurance Company AMB	Sep. 30, 2012 Berkley Insurance Company S&P	Sep. 30, 2012 Tower Insurance Company		Sep. 30, 2012 Tower Insurance Company AMB	Sep. 30, 2012 Tower Insurance Company S&P	Sep. 30, 2012 General Reinsurance Corporation		Sep. 30, 2012 General Reinsurance Corporation AMB	Sep. 30, 2012 General Reinsurance Corporation S&P	Sep. 30, 2012 Scor Global P&C SE		Sep. 30, 2012 Scor Global P&C SE AMB	Sep. 30, 2012 Scor Global P&C SE S&P	Sep. 30, 2012 Ace Property and Casualty Insurance Company	Sep. 30, 2012 Ace Property and Casualty Insurance Company AMB	Sep. 30, 2012 Ace Property and Casualty Insurance Company S&P	Sep. 30, 2012 Validus Reinsurance Ltd		Sep. 30, 2012 Validus Reinsurance Ltd AMB	Sep. 30, 2012 Validus Reinsurance Ltd S&P	Sep. 30, 2012 Scor Holding (Switzerland) AG		Sep. 30, 2012 Scor Holding (Switzerland) AG AMB	Sep. 30, 2012 Scor Holding (Switzerland) AG S&P	Sep. 30, 2012 Sirius America Insurance Company		Sep. 30, 2012 Sirius America Insurance Company AMB	Sep. 30, 2012 Sirius America Insurance Company S&P	Sep. 30, 2012 Platinum Underwriters Re		Sep. 30, 2012 Platinum Underwriters Re AMB	Sep. 30, 2012 Platinum Underwriters Re S&P	Sep. 30, 2012 Lloyd Syndicate #4000		Sep. 30, 2012 Lloyd Syndicate #4000 AMB	Sep. 30, 2012 Lloyd Syndicate #4000 S&P	Sep. 30, 2012 Munchener Ruckversicherungs-Gesellschaft		Sep. 30, 2012 Munchener Ruckversicherungs-Gesellschaft AMB	Sep. 30, 2012 Munchener Ruckversicherungs-Gesellschaft S&P	Sep. 30, 2012 AXIS Re Europe		Sep. 30, 2012 AXIS Re Europe AMB	Sep. 30, 2012 AXIS Re Europe S&P	Sep. 30, 2012 Insurance Companies		Sep. 30, 2012 All Other
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	$ 224,851		$ 164,162	$ 5,017				$ 10,356				$ 17,445				$ 18,256				$ 41,244				$ 10,053				$ 8,622				$ 9,500				$ 1,795				$ 10,844				$ 590				$ 13,158				$ 971			$ 2,866				$ 1,128				$ 75				$ 472				$ 2,657				$ 387				$ 3,563				$ 158,999		$ 65,852
Reinsurance Recoverables Paid/Unpaid Losses	876,423			91,035				85,215				72,832				71,308				34,388				36,191				33,044				20,987				26,003				15,038				25,129				12,287				20,919			18,706				17,177				18,086				17,488				14,177				14,331				10,904				655,245		221,178
Reinsurance Recoverables Total	1,101,274			96,052				95,571				90,277				89,564				75,632				46,244				41,666				30,487				27,798				25,882				25,719				25,445				21,890			21,572				18,305				18,161				17,960				16,834				14,718				14,467				814,244		287,030
Collateral Held	$ 237,084	[1]		$ 7,152	[1]			$ 1,235	[1]			$ 12,848	[1]			$ 8,209	[1]			$ 22,746	[1]			$ 19,814	[1]			$ 10,232	[1]			$ 4,085	[1]			$ 3,147	[1]			$ 4,129	[1]			$ 1,006	[1]			$ 7,452	[1]						$ 9,490	[1]			$ 4,254	[1]			$ 2,455	[1]			$ 1,552	[1]			$ 2,254	[1]			$ 6,115	[1]			$ 2,083	[1]			$ 130,258	[1]	$ 106,826	[1]
Rating						A+	AA-			A+	AA-			A	A+			A+	A+			A++	AA+			A+	A+			A	A+			A	A			A+	A+			A-	NR			A++	AA+			A	A+		A+	AA-			A	A			A	A+			A	A-			A	A-			A	A+			A+	AA-			A	A+

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