Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) (USD $) In Thousands, unless otherwise specified						6 Months Ended				6 Months Ended				6 Months Ended				6 Months Ended				6 Months Ended				6 Months Ended				6 Months Ended				6 Months Ended				6 Months Ended				6 Months Ended				6 Months Ended				6 Months Ended				6 Months Ended				6 Months Ended			6 Months Ended				6 Months Ended				6 Months Ended				6 Months Ended				6 Months Ended				6 Months Ended
	Jun. 30, 2012		Dec. 31, 2011	Jun. 30, 2012 Everest Reinsurance Company		Jun. 30, 2012 Everest Reinsurance Company AMB	Jun. 30, 2012 Everest Reinsurance Company S&P	Jun. 30, 2012 Munich Reinsurance America Inc.		Jun. 30, 2012 Munich Reinsurance America Inc. AMB	Jun. 30, 2012 Munich Reinsurance America Inc. S&P	Jun. 30, 2012 Swiss Reinsurance America Corporation		Jun. 30, 2012 Swiss Reinsurance America Corporation AMB	Jun. 30, 2012 Swiss Reinsurance America Corporation S&P	Jun. 30, 2012 Transatlantic Reinsurance Company		Jun. 30, 2012 Transatlantic Reinsurance Company AMB	Jun. 30, 2012 Transatlantic Reinsurance Company S&P	Jun. 30, 2012 National Indemnity Company		Jun. 30, 2012 National Indemnity Company AMB	Jun. 30, 2012 National Indemnity Company S&P	Jun. 30, 2012 Lloyd's Syndicate #2003		Jun. 30, 2012 Lloyd's Syndicate #2003 AMB	Jun. 30, 2012 Lloyd's Syndicate #2003 S&P	Jun. 30, 2012 Partner Reinsurance Europe		Jun. 30, 2012 Partner Reinsurance Europe AMB	Jun. 30, 2012 Partner Reinsurance Europe S&P	Jun. 30, 2012 Allied World Reinsurance		Jun. 30, 2012 Allied World Reinsurance AMB	Jun. 30, 2012 Allied World Reinsurance S&P	Jun. 30, 2012 Berkley Insurance Company		Jun. 30, 2012 Berkley Insurance Company AMB	Jun. 30, 2012 Berkley Insurance Company S&P	Jun. 30, 2012 General Reinsurance Corporation		Jun. 30, 2012 General Reinsurance Corporation AMB	Jun. 30, 2012 General Reinsurance Corporation S&P	Jun. 30, 2012 Scor Global P&C SE		Jun. 30, 2012 Scor Global P&C SE AMB	Jun. 30, 2012 Scor Global P&C SE S&P	Jun. 30, 2012 Tower Insurance Company		Jun. 30, 2012 Tower Insurance Company AMB	Jun. 30, 2012 Tower Insurance Company S&P	Jun. 30, 2012 Scor Holding (Switzerland) AG		Jun. 30, 2012 Scor Holding (Switzerland) AG AMB	Jun. 30, 2012 Scor Holding (Switzerland) AG S&P	Jun. 30, 2012 Validus Reinsurance Ltd		Jun. 30, 2012 Validus Reinsurance Ltd AMB	Jun. 30, 2012 Validus Reinsurance Ltd S&P	Jun. 30, 2012 Ace Property and Casualty Insurance Company	Jun. 30, 2012 Ace Property and Casualty Insurance Company AMB	Jun. 30, 2012 Ace Property and Casualty Insurance Company S&P	Jun. 30, 2012 Platinum Underwriters Re		Jun. 30, 2012 Platinum Underwriters Re AMB	Jun. 30, 2012 Platinum Underwriters Re S&P	Jun. 30, 2012 Munchener Ruckversicherungs-Gesellschaft		Jun. 30, 2012 Munchener Ruckversicherungs-Gesellschaft AMB	Jun. 30, 2012 Munchener Ruckversicherungs-Gesellschaft S&P	Jun. 30, 2012 Sirius America Insurance Company		Jun. 30, 2012 Sirius America Insurance Company AMB	Jun. 30, 2012 Sirius America Insurance Company S&P	Jun. 30, 2012 AXIS Re Europe		Jun. 30, 2012 AXIS Re Europe AMB	Jun. 30, 2012 AXIS Re Europe S&P	Jun. 30, 2012 Lloyd Syndicate #4000		Jun. 30, 2012 Lloyd Syndicate #4000 AMB	Jun. 30, 2012 Lloyd Syndicate #4000 S&P	Jun. 30, 2012 Insurance Companies		Jun. 30, 2012 All Other
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	$ 211,089		$ 164,162	$ 16,225				$ 11,108				$ 5,076				$ 15,570				$ 34,376				$ 8,092				$ 9,447				$ 8,108				$ 1,848				$ 675				$ 13,613				$ 10,331				$ 1,323				$ 2,979				$ 1,022			$ 558				$ 390				$ 88				$ 4,252				$ 2,499				$ 147,580		$ 63,509
Reinsurance Recoverables Paid/Unpaid Losses	886,577			75,837				78,824				84,310				73,065				37,716				32,416				31,013				21,459				27,099				26,487				11,644				14,166				20,988				18,560				19,824			18,884				19,037				19,205				13,187				14,341				658,062		228,515
Reinsurance Recoverables Total	1,097,666			92,062				89,932				89,386				88,635				72,092				40,508				40,460				29,567				28,947				27,162				25,257				24,497				22,311				21,539				20,846			19,442				19,427				19,293				17,439				16,840				805,642		292,024
Collateral Held	$ 211,869	[1]		$ 6,179	[1]			$ 222	[1]			$ 6,556	[1]			$ 6,745	[1]			$ 15,981	[1]			$ 8,245	[1]			$ 22,540	[1]			$ 2,405	[1]			$ 596	[1]			$ 890	[1]			$ 9,190	[1]			$ 4,541	[1]			$ 9,069	[1]			$ 9,745	[1]						$ 1,994	[1]			$ 6,719	[1]			$ 335	[1]			$ 5,742	[1]			$ 2,216	[1]			$ 119,910	[1]	$ 91,959	[1]
Rating						A+	A+			A+	AA-			A+	AA-			A	A+			A++	AA+			A	A+			A+	A+			A	A			A+	A+			A++	AA+			A	A+			A-	NR			A	A+			A	A-		A+	AA-			A	A-			A+	AA-			A	A-			A	A+			A	A+

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