Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Dec. 31, 2012
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	$ 232,776		$ 221,015
Reinsurance Recoverables Paid/Unpaid Losses	924,245
Reinsurance Recoverables Total	1,157,021	[1]
Collateral Held	225,905	[2]
Rating	AA
National Indemnity Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	48,252
Reinsurance Recoverables Paid/Unpaid Losses	61,076
Reinsurance Recoverables Total	109,328	[1]
Collateral Held	32,952	[2]
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	5,551
Reinsurance Recoverables Paid/Unpaid Losses	87,274
Reinsurance Recoverables Total	92,825	[1]
Collateral Held	6,070	[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	19,321
Reinsurance Recoverables Paid/Unpaid Losses	71,906
Reinsurance Recoverables Total	91,227	[1]
Collateral Held	9,739	[2]
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	20,648
Reinsurance Recoverables Paid/Unpaid Losses	70,431
Reinsurance Recoverables Total	91,079	[1]
Collateral Held	9,102	[2]
Everest Reinsurance Company | AMB
Ceded Credit Risk [Line Items]
Rating	A+
Everest Reinsurance Company | S&P
Ceded Credit Risk [Line Items]
Rating	A+
Munich Reinsurance America Inc.
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	9,613
Reinsurance Recoverables Paid/Unpaid Losses	78,428
Reinsurance Recoverables Total	88,041	[1]
Collateral Held	3,329	[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-
Lloyd Syndicate #2003
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	8,859
Reinsurance Recoverables Paid/Unpaid Losses	38,850
Reinsurance Recoverables Total	47,709	[1]
Collateral Held	10,251	[2]
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	8,787
Reinsurance Recoverables Paid/Unpaid Losses	34,121
Reinsurance Recoverables Total	42,908	[1]
Collateral Held	17,457	[2]
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	11,812
Reinsurance Recoverables Paid/Unpaid Losses	26,486
Reinsurance Recoverables Total	38,298	[1]
Collateral Held	4,682	[2]
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	10,065
Reinsurance Recoverables Paid/Unpaid Losses	21,576
Reinsurance Recoverables Total	31,641	[1]
Collateral Held	6,829	[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+
Tower Insurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	11,993
Reinsurance Recoverables Paid/Unpaid Losses	17,045
Reinsurance Recoverables Total	29,038	[1]
Collateral Held	8,227	[2]
Tower Insurance Company | AMB
Ceded Credit Risk [Line Items]
Rating	A-
Tower Insurance Company | S&P
Ceded Credit Risk [Line Items]
Rating	NR
General Reinsurance Corporation
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	268
Reinsurance Recoverables Paid/Unpaid Losses	26,144
Reinsurance Recoverables Total	26,412	[1]
Collateral Held	731	[2]
General Reinsurance Corporation | AMB
Ceded Credit Risk [Line Items]
Rating	A++
General Reinsurance Corporation | S&P
Ceded Credit Risk [Line Items]
Rating	AA+
Validus Reinsurance Ltd
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	2,739
Reinsurance Recoverables Paid/Unpaid Losses	22,892
Reinsurance Recoverables Total	25,631	[1]
Collateral Held	13,041	[2]
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	1,556
Reinsurance Recoverables Paid/Unpaid Losses	19,144
Reinsurance Recoverables Total	20,700	[1]
Collateral Held	241	[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	790
Reinsurance Recoverables Paid/Unpaid Losses	16,810
Reinsurance Recoverables Total	17,600	[1]
Collateral Held	8,214	[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+
Atlantic Specialty Insurance
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	7,843
Reinsurance Recoverables Paid/Unpaid Losses	8,295
Reinsurance Recoverables Total	16,138	[1]
Collateral Held	3,489	[2]
Atlantic Specialty Insurance | AMB
Ceded Credit Risk [Line Items]
Rating	A
Atlantic Specialty Insurance | S&P
Ceded Credit Risk [Line Items]
Rating	A-
Ace Property and Casualty Insurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	801
Reinsurance Recoverables Paid/Unpaid Losses	14,780
Reinsurance Recoverables Total	15,581	[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-
AXIS Re Europe
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	2,995
Reinsurance Recoverables Paid/Unpaid Losses	11,782
Reinsurance Recoverables Total	14,777	[1]
Collateral Held	2,867	[2]
AXIS Re Europe | AMB
Ceded Credit Risk [Line Items]
Rating	A
AXIS Re Europe | S&P
Ceded Credit Risk [Line Items]
Rating	A+
Platinum Underwriters Re
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	354
Reinsurance Recoverables Paid/Unpaid Losses	14,280
Reinsurance Recoverables Total	14,634	[1]
Collateral Held	1,851	[2]
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	1,403
Reinsurance Recoverables Paid/Unpaid Losses	12,168
Reinsurance Recoverables Total	13,571	[1]
Collateral Held	882	[2]
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	5,296
Reinsurance Recoverables Paid/Unpaid Losses	8,159
Reinsurance Recoverables Total	13,455	[1]
Collateral Held	1,348	[2]
Star Insurance Company | AMB
Ceded Credit Risk [Line Items]
Rating	A-
Star Insurance Company | S&P
Ceded Credit Risk [Line Items]
Rating	BBB
Insurance
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	178,946
Reinsurance Recoverables Paid/Unpaid Losses	661,647
Reinsurance Recoverables Total	840,593	[1]
Collateral Held	141,302	[2]
All Other
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	53,830
Reinsurance Recoverables Paid/Unpaid Losses	262,598
Reinsurance Recoverables Total	316,428	[1]
Collateral Held	$ 84,603	[2]

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

[2] Collateral of $225.9 million consists of $165.1 million in ceded balances payable, $57.2 million in letters of credit, and $3.6 million of other balances held by the Company's Insurance Companies and Lloyd's Operations.