Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	$ 164,162		$ 156,869
Reinsurance Recoverables Paid/Unpaid Losses	889,236
Reinsurance Recoverables Total	1,053,398	[1]
Collateral Held	177,431	[2]
Rating	AA
Munich Reinsurance America Inc.
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	11,286
Reinsurance Recoverables Paid/Unpaid Losses	78,562
Reinsurance Recoverables Total	89,848	[1]
Collateral Held	386	[2]
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-
Everest Reinsurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	14,528
Reinsurance Recoverables Paid/Unpaid Losses	75,094
Reinsurance Recoverables Total	89,622	[1]
Collateral Held	6,634	[2]
Everest Reinsurance Company | AMB
Ceded Credit Risk [Line Items]
Rating	A+
Everest Reinsurance Company | S&P
Ceded Credit Risk [Line Items]
Rating	A+
Swiss Reinsurance America Corporation
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	4,730
Reinsurance Recoverables Paid/Unpaid Losses	84,343
Reinsurance Recoverables Total	89,073	[1]
Collateral Held	8,041	[2]
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-
Transatlantic Reinsurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	15,177
Reinsurance Recoverables Paid/Unpaid Losses	71,530
Reinsurance Recoverables Total	86,707	[1]
Collateral Held	6,416	[2]
Transatlantic Reinsurance Company | AMB
Ceded Credit Risk [Line Items]
Rating	A
Transatlantic Reinsurance Company | S&P
Ceded Credit Risk [Line Items]
Rating	A+
National Indemnity Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	15,591
Reinsurance Recoverables Paid/Unpaid Losses	37,251
Reinsurance Recoverables Total	52,842	[1]
Collateral Held	5,774	[2]
National Indemnity Company | AMB
Ceded Credit Risk [Line Items]
Rating	A++
National Indemnity Company | S&P
Ceded Credit Risk [Line Items]
Rating	AA+
Partner Reinsurance Europe
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	7,227
Reinsurance Recoverables Paid/Unpaid Losses	32,682
Reinsurance Recoverables Total	39,909	[1]
Collateral Held	20,272	[2]
Partner Reinsurance Europe | AMB
Ceded Credit Risk [Line Items]
Rating	A+
Partner Reinsurance Europe | S&P
Ceded Credit Risk [Line Items]
Rating	AA-
Berkley Insurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	1,755
Reinsurance Recoverables Paid/Unpaid Losses	31,278
Reinsurance Recoverables Total	33,033	[1]
Collateral Held	243	[2]
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	2,234
Reinsurance Recoverables Paid/Unpaid Losses	29,822
Reinsurance Recoverables Total	32,056	[1]
Collateral Held	7,754	[2]
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
Lloyd's Syndicate #2003
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	5,581
Reinsurance Recoverables Paid/Unpaid Losses	25,380
Reinsurance Recoverables Total	30,961	[1]
Collateral Held	6,481	[2]
Lloyd's Syndicate #2003 | AMB
Ceded Credit Risk [Line Items]
Rating	A
Lloyd's Syndicate #2003 | S&P
Ceded Credit Risk [Line Items]
Rating	A+
General Reinsurance Corporation
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	703
Reinsurance Recoverables Paid/Unpaid Losses	27,630
Reinsurance Recoverables Total	28,333	[1]
Collateral Held	852	[2]
General Reinsurance Corporation | AMB
Ceded Credit Risk [Line Items]
Rating	A++
General Reinsurance Corporation | S&P
Ceded Credit Risk [Line Items]
Rating	AA+
Allied World Reinsurance
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	6,773
Reinsurance Recoverables Paid/Unpaid Losses	20,291
Reinsurance Recoverables Total	27,064	[1]
Collateral Held	2,293	[2]
Allied World Reinsurance | AMB
Ceded Credit Risk [Line Items]
Rating	A
Allied World Reinsurance | S&P
Ceded Credit Risk [Line Items]
Rating	A
Munchener Ruckversicherungs-Gesellschaft
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	858
Reinsurance Recoverables Paid/Unpaid Losses	25,815
Reinsurance Recoverables Total	26,673	[1]
Collateral Held	6,614	[2]
Munchener Ruckversicherungs-Gesellschaft | AMB
Ceded Credit Risk [Line Items]
Rating	A+
Munchener Ruckversicherungs-Gesellschaft | S&P
Ceded Credit Risk [Line Items]
Rating	AA-
Ace Property and Casualty Insurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	1,525
Reinsurance Recoverables Paid/Unpaid Losses	22,339
Reinsurance Recoverables Total	23,864	[1]
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-
Platinum Underwriters Re
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	800
Reinsurance Recoverables Paid/Unpaid Losses	22,186
Reinsurance Recoverables Total	22,986	[1]
Collateral Held	2,829	[2]
Platinum Underwriters Re | AMB
Ceded Credit Risk [Line Items]
Rating	A
Platinum Underwriters Re | S&P
Ceded Credit Risk [Line Items]
Rating	A-
White Mountains Reinsurance of America
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	117
Reinsurance Recoverables Paid/Unpaid Losses	22,544
Reinsurance Recoverables Total	22,661	[1]
Collateral Held	96	[2]
White Mountains Reinsurance of America | AMB
Ceded Credit Risk [Line Items]
Rating	A
White Mountains Reinsurance of America | S&P
Ceded Credit Risk [Line Items]
Rating	A-
AXIS Re Europe
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	6,188
Reinsurance Recoverables Paid/Unpaid Losses	15,853
Reinsurance Recoverables Total	22,041	[1]
Collateral Held	5,434	[2]
AXIS Re Europe | AMB
Ceded Credit Risk [Line Items]
Rating	A
AXIS Re Europe | S&P
Ceded Credit Risk [Line Items]
Rating	A+
Tower Insurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	8,191
Reinsurance Recoverables Paid/Unpaid Losses	12,980
Reinsurance Recoverables Total	21,171	[1]
Collateral Held	1,798	[2]
Tower Insurance Company | AMB
Ceded Credit Risk [Line Items]
Rating	A-
Tower Insurance Company | S&P
Ceded Credit Risk [Line Items]
Rating	NR
Scor Global P&C SE
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	10,871
Reinsurance Recoverables Paid/Unpaid Losses	5,911
Reinsurance Recoverables Total	16,782	[1]
Collateral Held	6,002	[2]
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
Lloyd Syndicate #4000
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	1,999
Reinsurance Recoverables Paid/Unpaid Losses	13,571
Reinsurance Recoverables Total	15,570	[1]
Collateral Held	1,814	[2]
Lloyd Syndicate #4000 | AMB
Ceded Credit Risk [Line Items]
Rating	A
Lloyd Syndicate #4000 | S&P
Ceded Credit Risk [Line Items]
Rating	A+
Validus Reinsurance Ltd
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	3,004
Reinsurance Recoverables Paid/Unpaid Losses	12,457
Reinsurance Recoverables Total	15,461	[1]
Collateral Held	9,981	[2]
Validus Reinsurance Ltd | AMB
Ceded Credit Risk [Line Items]
Rating	A-
Validus Reinsurance Ltd | S&P
Ceded Credit Risk [Line Items]
Rating	A-
Insurance Companies
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	119,138
Reinsurance Recoverables Paid/Unpaid Losses	667,519
Reinsurance Recoverables Total	786,657	[1]
Collateral Held	99,714	[2]
All Other
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	45,024
Reinsurance Recoverables Paid/Unpaid Losses	221,717
Reinsurance Recoverables Total	266,741	[1]
Collateral Held	$ 77,717	[2]

[1] Net of reserve for uncollectible reinsurance of approximately $13.0 million.

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