Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	$ 221,015		$ 164,162
Reinsurance Recoverables Paid/Unpaid Losses	929,421
Reinsurance Recoverables Total	1,150,436	[1],[2],[3]
Collateral Held	227,165	[4]
Rating	AA
National Indemnity Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	41,004
Reinsurance Recoverables Paid/Unpaid Losses	58,215
Reinsurance Recoverables Total	99,219	[3]
Collateral Held	27,976	[4]
National Indemnity Company | AMB
Ceded Credit Risk [Line Items]
Rating	A++
National Indemnity Company | S&P
Ceded Credit Risk [Line Items]
Rating	AA+
Swiss Reinsurance America Corporation
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	4,708
Reinsurance Recoverables Paid/Unpaid Losses	89,842
Reinsurance Recoverables Total	94,550	[3]
Collateral Held	9,916	[4]
Swiss Reinsurance America Corporation | AMB
Ceded Credit Risk [Line Items]
Rating	A+
Swiss Reinsurance America Corporation | S&P
Ceded Credit Risk [Line Items]
Rating	AA-
Munich Reinsurance America Inc.
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	10,005
Reinsurance Recoverables Paid/Unpaid Losses	81,982
Reinsurance Recoverables Total	91,987	[3]
Collateral Held	5,356	[4]
Munich Reinsurance America Inc. | AMB
Ceded Credit Risk [Line Items]
Rating	A+
Munich Reinsurance America Inc. | S&P
Ceded Credit Risk [Line Items]
Rating	AA-
Transatlantic Reinsurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	17,763
Reinsurance Recoverables Paid/Unpaid Losses	73,221
Reinsurance Recoverables Total	90,984	[3]
Collateral Held	8,219	[4]
Transatlantic Reinsurance Company | AMB
Ceded Credit Risk [Line Items]
Rating	A
Transatlantic Reinsurance Company | S&P
Ceded Credit Risk [Line Items]
Rating	A+
Everest Reinsurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	19,098
Reinsurance Recoverables Paid/Unpaid Losses	70,097
Reinsurance Recoverables Total	89,195	[3]
Collateral Held	8,003	[4]
Everest Reinsurance Company | AMB
Ceded Credit Risk [Line Items]
Rating	A+
Everest Reinsurance Company | S&P
Ceded Credit Risk [Line Items]
Rating	A+
Lloyd Syndicate #2003
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	7,946
Reinsurance Recoverables Paid/Unpaid Losses	38,757
Reinsurance Recoverables Total	46,703	[3]
Collateral Held	10,909	[4]
Lloyd Syndicate #2003 | AMB
Ceded Credit Risk [Line Items]
Rating	A
Lloyd Syndicate #2003 | S&P
Ceded Credit Risk [Line Items]
Rating	A+
Partner Reinsurance Europe
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	9,811
Reinsurance Recoverables Paid/Unpaid Losses	34,859
Reinsurance Recoverables Total	44,670	[3]
Collateral Held	17,168	[4]
Partner Reinsurance Europe | AMB
Ceded Credit Risk [Line Items]
Rating	A+
Partner Reinsurance Europe | S&P
Ceded Credit Risk [Line Items]
Rating	A+
Allied World Reinsurance
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	10,017
Reinsurance Recoverables Paid/Unpaid Losses	24,749
Reinsurance Recoverables Total	34,766	[3]
Collateral Held	3,577	[4]
Allied World Reinsurance | AMB
Ceded Credit Risk [Line Items]
Rating	A
Allied World Reinsurance | S&P
Ceded Credit Risk [Line Items]
Rating	A
Scor Global P&C SE
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	12,521
Reinsurance Recoverables Paid/Unpaid Losses	21,505
Reinsurance Recoverables Total	34,026	[3]
Collateral Held	7,139	[4]
Scor Global P&C SE | AMB
Ceded Credit Risk [Line Items]
Rating	A
Scor Global P&C SE | S&P
Ceded Credit Risk [Line Items]
Rating	A+
General Reinsurance Corporation
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	468
Reinsurance Recoverables Paid/Unpaid Losses	27,134
Reinsurance Recoverables Total	27,602	[3]
Collateral Held	789	[4]
General Reinsurance Corporation | AMB
Ceded Credit Risk [Line Items]
Rating	A++
General Reinsurance Corporation | S&P
Ceded Credit Risk [Line Items]
Rating	AA+
Tower Insurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	11,363
Reinsurance Recoverables Paid/Unpaid Losses	15,271
Reinsurance Recoverables Total	26,634	[3]
Collateral Held	7,195	[4]
Tower Insurance Company | AMB
Ceded Credit Risk [Line Items]
Rating	A-
Tower Insurance Company | S&P
Ceded Credit Risk [Line Items]
Rating	NR
Validus Reinsurance Ltd
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	2,719
Reinsurance Recoverables Paid/Unpaid Losses	22,357
Reinsurance Recoverables Total	25,076	[3]
Collateral Held	13,830	[4]
Validus Reinsurance Ltd | AMB
Ceded Credit Risk [Line Items]
Rating	A
Validus Reinsurance Ltd | S&P
Ceded Credit Risk [Line Items]
Rating	A
Berkley Insurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	1,675
Reinsurance Recoverables Paid/Unpaid Losses	19,402
Reinsurance Recoverables Total	21,077	[3]
Collateral Held	88	[4]
Berkley Insurance Company | AMB
Ceded Credit Risk [Line Items]
Rating	A+
Berkley Insurance Company | S&P
Ceded Credit Risk [Line Items]
Rating	A+
Scor Holding (Switzerland) AG
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	877
Reinsurance Recoverables Paid/Unpaid Losses	16,139
Reinsurance Recoverables Total	17,016	[3]
Collateral Held	9,080	[4]
Scor Holding (Switzerland) AG | AMB
Ceded Credit Risk [Line Items]
Rating	A
Scor Holding (Switzerland) AG | S&P
Ceded Credit Risk [Line Items]
Rating	A+
Ace Property and Casualty Insurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	867
Reinsurance Recoverables Paid/Unpaid Losses	15,979
Reinsurance Recoverables Total	16,846	[3]
Ace Property and Casualty Insurance Company | AMB
Ceded Credit Risk [Line Items]
Rating	A+
Ace Property and Casualty Insurance Company | S&P
Ceded Credit Risk [Line Items]
Rating	AA-
AXIS Re Europe
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	3,084
Reinsurance Recoverables Paid/Unpaid Losses	12,566
Reinsurance Recoverables Total	15,650	[3]
Collateral Held	2,530	[4]
AXIS Re Europe | AMB
Ceded Credit Risk [Line Items]
Rating	A
AXIS Re Europe | S&P
Ceded Credit Risk [Line Items]
Rating	A+
Sirius America Insurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	59
Reinsurance Recoverables Paid/Unpaid Losses	15,457
Reinsurance Recoverables Total	15,516	[3]
Collateral Held	21	[4]
Sirius America Insurance Company | AMB
Ceded Credit Risk [Line Items]
Rating	A
Sirius America Insurance Company | S&P
Ceded Credit Risk [Line Items]
Rating	A-
Platinum Underwriters Re
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	405
Reinsurance Recoverables Paid/Unpaid Losses	14,653
Reinsurance Recoverables Total	15,058	[3]
Collateral Held	1,594	[4]
Platinum Underwriters Re | AMB
Ceded Credit Risk [Line Items]
Rating	A
Platinum Underwriters Re | S&P
Ceded Credit Risk [Line Items]
Rating	A-
Lloyd Syndicate #4000
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	2,407
Reinsurance Recoverables Paid/Unpaid Losses	12,323
Reinsurance Recoverables Total	14,730	[3]
Collateral Held	1,772	[4]
Lloyd Syndicate #4000 | AMB
Ceded Credit Risk [Line Items]
Rating	A
Lloyd Syndicate #4000 | S&P
Ceded Credit Risk [Line Items]
Rating	A+
Star Insurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	7,812
Reinsurance Recoverables Paid/Unpaid Losses	6,830
Reinsurance Recoverables Total	14,642	[3]
Collateral Held	2,837	[4]
Star Insurance Company | AMB
Ceded Credit Risk [Line Items]
Rating	A-
Star Insurance Company | S&P
Ceded Credit Risk [Line Items]
Rating	BBB
Insurance Companies
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	164,609
Reinsurance Recoverables Paid/Unpaid Losses	671,338
Reinsurance Recoverables Total	835,947	[3]
Collateral Held	137,999	[4]
All Other
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	56,406
Reinsurance Recoverables Paid/Unpaid Losses	258,083
Reinsurance Recoverables Total	314,489	[3]
Collateral Held	$ 89,166	[4]

[1] Equivalent S&P rating used for certain companies when an A.M. Best rating was unavailable.

[2] Net of reserve for uncollectible reinsurance of approximately $11.5 million. The carrying value consists of reinsurance recoverables on paid losses due within 30-45 days and reinsurance on unpaid losses which by nature of our reserving process is our best estimate of the value as of December 31, 2012.

[3] Net of reserve for uncollectible reinsurance of approximately $11.5 million.

[4] Collateral of $227.2 million consists of $165.8 million in ceded balances payable, $57.2 million in letters of credit, and $4.2 million of other balances held by the Company's Insurance Companies and Lloyd's Operations.