Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) (USD $) In Thousands, unless otherwise specified	3 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				6 Months Ended
	Jun. 30, 2013		Dec. 31, 2012	Jun. 30, 2013 National Indemnity Company		Jun. 30, 2013 National Indemnity Company AMB	Jun. 30, 2013 National Indemnity Company S&P	Jun. 30, 2013 Swiss Reinsurance America Corporation		Jun. 30, 2013 Swiss Reinsurance America Corporation AMB	Jun. 30, 2013 Swiss Reinsurance America Corporation S&P	Jun. 30, 2013 Transatlantic Reinsurance Company		Jun. 30, 2013 Transatlantic Reinsurance Company AMB	Jun. 30, 2013 Transatlantic Reinsurance Company S&P	Jun. 30, 2013 Everest Reinsurance Company		Jun. 30, 2013 Everest Reinsurance Company AMB	Jun. 30, 2013 Everest Reinsurance Company S&P	Jun. 30, 2013 Munich Reinsurance America Inc.		Jun. 30, 2013 Munich Reinsurance America Inc. AMB	Jun. 30, 2013 Munich Reinsurance America Inc. S&P	Jun. 30, 2013 Lloyd Syndicate #2003		Jun. 30, 2013 Lloyd Syndicate #2003 AMB	Jun. 30, 2013 Lloyd Syndicate #2003 S&P	Jun. 30, 2013 Partner Reinsurance Europe		Jun. 30, 2013 Partner Reinsurance Europe AMB	Jun. 30, 2013 Partner Reinsurance Europe S&P	Jun. 30, 2013 Scor Global P&C SE		Jun. 30, 2013 Scor Global P&C SE AMB	Jun. 30, 2013 Scor Global P&C SE S&P	Jun. 30, 2013 Allied World Reinsurance		Jun. 30, 2013 Allied World Reinsurance AMB	Jun. 30, 2013 Allied World Reinsurance S&P	Jun. 30, 2013 Tower Insurance Company		Jun. 30, 2013 Tower Insurance Company AMB	Jun. 30, 2013 Tower Insurance Company S&P	Jun. 30, 2013 General Reinsurance Corporation		Jun. 30, 2013 General Reinsurance Corporation AMB	Jun. 30, 2013 General Reinsurance Corporation S&P	Jun. 30, 2013 Validus Reinsurance Ltd		Jun. 30, 2013 Validus Reinsurance Ltd AMB	Jun. 30, 2013 Validus Reinsurance Ltd S&P	Jun. 30, 2013 Berkley Insurance Company		Jun. 30, 2013 Berkley Insurance Company AMB	Jun. 30, 2013 Berkley Insurance Company S&P	Jun. 30, 2013 Scor Holding (Switzerland) AG		Jun. 30, 2013 Scor Holding (Switzerland) AG AMB	Jun. 30, 2013 Scor Holding (Switzerland) AG S&P	Jun. 30, 2013 AXIS Re Europe		Jun. 30, 2013 AXIS Re Europe AMB	Jun. 30, 2013 AXIS Re Europe S&P	Jun. 30, 2013 Atlantic Specialty Insurance		Jun. 30, 2013 Atlantic Specialty Insurance AMB	Jun. 30, 2013 Atlantic Specialty Insurance S&P	Jun. 30, 2013 Employers Mutual Casualty Company		Jun. 30, 2013 Employers Mutual Casualty Company AMB	Jun. 30, 2013 Employers Mutual Casualty Company S&P	Jun. 30, 2013 Platinum Underwriters Re		Jun. 30, 2013 Platinum Underwriters Re AMB	Jun. 30, 2013 Platinum Underwriters Re S&P	Jun. 30, 2013 Ace Property and Casualty Insurance Company		Jun. 30, 2013 Ace Property and Casualty Insurance Company AMB	Jun. 30, 2013 Ace Property and Casualty Insurance Company S&P	Jun. 30, 2013 QBE Reinsurance Corp		Jun. 30, 2013 QBE Reinsurance Corp AMB	Jun. 30, 2013 QBE Reinsurance Corp S&P	Jun. 30, 2013 Insurance		Jun. 30, 2013 All Other
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	$ 251,046		$ 221,015	$ 53,397				$ 5,949				$ 20,597				$ 21,219				$ 7,923				$ 10,938				$ 10,485				$ 11,642				$ 12,803				$ 13,397				$ 194				$ 2,695				$ 1,653				$ 691				$ 4,182				$ 8,658				$ 10,867				$ 304				$ 730				$ 6,362				$ 204,686		$ 46,360
Reinsurance Recoverables Paid/Unpaid Losses	934,531			64,605				86,684				70,872				69,516				80,115				39,381				37,600				27,768				26,519				16,537				25,967				22,860				19,389				18,159				13,102				7,550				5,317				13,899				13,297				6,950				666,087		268,444
Reinsurance Recoverables Total	1,185,577	[1]		118,002	[1]			92,633	[1]			91,469	[1]			90,735	[1]			88,038	[1]			50,319	[1]			48,085	[1]			39,410	[1]			39,322	[1]			29,934	[1]			26,161	[1]			25,555	[1]			21,042	[1]			18,850	[1]			17,284	[1]			16,208	[1]			16,184	[1]			14,203	[1]			14,027	[1]			13,312	[1]			870,773	[1]	314,804	[1]
Collateral Held	$ 243,872	[2]		$ 35,018	[2]			$ 7,685	[2]			$ 8,971	[2]			$ 8,025	[2]			$ 4,133	[2]			$ 12,490	[2]			$ 20,108	[2]			$ 10,455	[2]			$ 3,872	[2]			$ 8,462	[2]			$ 1,092	[2]			$ 12,884	[2]			$ 405	[2]			$ 8,681	[2]			$ 4,718	[2]			$ 3,102	[2]			$ 10,557	[2]			$ 1,696	[2]							$ 2,094	[2]			$ 164,448	[2]	$ 79,424	[2]
Rating	AA					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+	A+			A	A+			A	A+			A	A-			A	NR			A	A-			A+	AA-			A	A+

[1] Net of reserve for uncollectible reinsurance of approximately $10.8 million.

[2] Collateral of $243.9 million consists of $183.7 million in ceded balances payable, $57.2 million in letters of credit, and $3.0 million of other balances held by the Company's Insurance Companies and Lloyd's Operations.