Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Dec. 31, 2011
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	$ 172,216		$ 164,162
Reinsurance Recoverables Paid/Unpaid Losses	908,414
Reinsurance Recoverables Total	1,080,630
Collateral Held	198,886	[1]
Rating	AA
Munich Reinsurance America Inc.
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	10,301
Reinsurance Recoverables Paid/Unpaid Losses	82,421
Reinsurance Recoverables Total	92,722
Collateral Held	15	[1]
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	14,229
Reinsurance Recoverables Paid/Unpaid Losses	77,070
Reinsurance Recoverables Total	91,299
Collateral Held	6,875	[1]
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	5,248
Reinsurance Recoverables Paid/Unpaid Losses	86,241
Reinsurance Recoverables Total	91,489
Collateral Held	5,984	[1]
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	14,335
Reinsurance Recoverables Paid/Unpaid Losses	74,038
Reinsurance Recoverables Total	88,373
Collateral Held	7,103	[1]
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	22,240
Reinsurance Recoverables Paid/Unpaid Losses	38,064
Reinsurance Recoverables Total	60,304
Collateral Held	12,718	[1]
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	6,883
Reinsurance Recoverables Paid/Unpaid Losses	31,065
Reinsurance Recoverables Total	37,948
Collateral Held	20,944	[1]
Partner Reinsurance Europe | AMB
Ceded Credit Risk [Line Items]
Rating	A+
Partner Reinsurance Europe | S&P
Ceded Credit Risk [Line Items]
Rating	A+
Lloyd's Syndicate #2003
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	6,344
Reinsurance Recoverables Paid/Unpaid Losses	32,108
Reinsurance Recoverables Total	38,452
Collateral Held	7,052	[1]
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+
Berkley Insurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	1,658
Reinsurance Recoverables Paid/Unpaid Losses	29,647
Reinsurance Recoverables Total	31,305
Collateral Held	423	[1]
Berkley Insurance Company | AMB
Ceded Credit Risk [Line Items]
Rating	A+
Berkley Insurance Company | S&P
Ceded Credit Risk [Line Items]
Rating	A+
Allied World Reinsurance
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	6,934
Reinsurance Recoverables Paid/Unpaid Losses	21,672
Reinsurance Recoverables Total	28,606
Collateral Held	2,776	[1]
Allied World Reinsurance | AMB
Ceded Credit Risk [Line Items]
Rating	A
Allied World Reinsurance | S&P
Ceded Credit Risk [Line Items]
Rating	A
General Reinsurance Corporation
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	743
Reinsurance Recoverables Paid/Unpaid Losses	25,836
Reinsurance Recoverables Total	26,579
Collateral Held	3,708	[1]
General Reinsurance Corporation | AMB
Ceded Credit Risk [Line Items]
Rating	A++
General Reinsurance Corporation | S&P
Ceded Credit Risk [Line Items]
Rating	AA+
Scor Holding (Switzerland) AG
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	1,696
Reinsurance Recoverables Paid/Unpaid Losses	24,742
Reinsurance Recoverables Total	26,438
Collateral Held	8,527	[1]
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	1,155
Reinsurance Recoverables Paid/Unpaid Losses	21,533
Reinsurance Recoverables Total	22,688
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-
Sirius America Insurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	114
Reinsurance Recoverables Paid/Unpaid Losses	22,373
Reinsurance Recoverables Total	22,487
Collateral Held	279	[1]
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-
Tower Insurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	8,202
Reinsurance Recoverables Paid/Unpaid Losses	13,006
Reinsurance Recoverables Total	21,208
Collateral Held	3,154	[1]
Tower Insurance Company | AMB
Ceded Credit Risk [Line Items]
Rating	A-
Tower Insurance Company | S&P
Ceded Credit Risk [Line Items]
Rating	NR
Platinum Underwriters Re
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	673
Reinsurance Recoverables Paid/Unpaid Losses	20,106
Reinsurance Recoverables Total	20,779
Collateral Held	2,305	[1]
Platinum Underwriters Re | AMB
Ceded Credit Risk [Line Items]
Rating	A
Platinum Underwriters Re | S&P
Ceded Credit Risk [Line Items]
Rating	A-
Validus Reinsurance Ltd
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	2,654
Reinsurance Recoverables Paid/Unpaid Losses	19,364
Reinsurance Recoverables Total	22,018
Collateral Held	12,075	[1]
Validus Reinsurance Ltd | AMB
Ceded Credit Risk [Line Items]
Rating	A
Validus Reinsurance Ltd | S&P
Ceded Credit Risk [Line Items]
Rating	A-
Munchener Ruckversicherungs-Gesellschaft
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	520
Reinsurance Recoverables Paid/Unpaid Losses	19,913
Reinsurance Recoverables Total	20,433
Collateral Held	6,716	[1]
Munchener Ruckversicherungs-Gesellschaft | AMB
Ceded Credit Risk [Line Items]
Rating	A+
Munchener Ruckversicherungs-Gesellschaft | S&P
Ceded Credit Risk [Line Items]
Rating	AA-
AXIS Re Europe
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	4,687
Reinsurance Recoverables Paid/Unpaid Losses	13,835
Reinsurance Recoverables Total	18,522
Collateral Held	5,246	[1]
AXIS Re Europe | AMB
Ceded Credit Risk [Line Items]
Rating	A
AXIS Re Europe | S&P
Ceded Credit Risk [Line Items]
Rating	A+
Lloyd Syndicate #4000
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	2,000
Reinsurance Recoverables Paid/Unpaid Losses	14,172
Reinsurance Recoverables Total	16,172
Collateral Held	1,828	[1]
Lloyd Syndicate #4000 | AMB
Ceded Credit Risk [Line Items]
Rating	A
Lloyd Syndicate #4000 | S&P
Ceded Credit Risk [Line Items]
Rating	A+
Scor Global P&C SE
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	7,455
Reinsurance Recoverables Paid/Unpaid Losses	8,381
Reinsurance Recoverables Total	15,836
Collateral Held	3,799	[1]
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
Insurance Companies
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	118,071
Reinsurance Recoverables Paid/Unpaid Losses	675,587
Reinsurance Recoverables Total	793,658
Collateral Held	111,527	[1]
All Others
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	54,145
Reinsurance Recoverables Paid/Unpaid Losses	232,827
Reinsurance Recoverables Total	286,972
Collateral Held	$ 87,359	[1]

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