Lists of 20 Largest Reinsurers Measured by Reinsurance Recoverable (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	$ 253,435		$ 221,015
Reinsurance Recoverables Paid/Unpaid Losses	841,244
Reinsurance Recoverables Total	1,094,679	[1]
Collateral Held	240,576	[2]
Rating	AA
National Indemnity Company | AM Best, A++ Rating | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	56,161
Reinsurance Recoverables Paid/Unpaid Losses	73,815
Reinsurance Recoverables Total	129,976	[1]
Collateral Held	37,385	[2]
National Indemnity Company | AM Best | AM Best, A++ Rating | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Rating	A++
National Indemnity Company | Standard & Poor's | AM Best, A++ Rating | Standard & Poor's, AA+ Rating
Ceded Credit Risk [Line Items]
Rating	AA+
Transatlantic Reinsurance Company | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	21,964
Reinsurance Recoverables Paid/Unpaid Losses	70,838
Reinsurance Recoverables Total	92,802	[1]
Collateral Held	10,052	[2]
Transatlantic Reinsurance Company | AM Best | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A
Transatlantic Reinsurance Company | Standard & Poor's | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Everest Reinsurance Company | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	22,082
Reinsurance Recoverables Paid/Unpaid Losses	64,313
Reinsurance Recoverables Total	86,395	[1]
Collateral Held	9,115	[2]
Everest Reinsurance Company | AM Best | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Everest Reinsurance Company | Standard & Poor's | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Swiss Reinsurance America Corporation | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	6,094
Reinsurance Recoverables Paid/Unpaid Losses	76,895
Reinsurance Recoverables Total	82,989	[1]
Collateral Held	8,638	[2]
Swiss Reinsurance America Corporation | AM Best | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating	A+
Swiss Reinsurance America Corporation | Standard & Poor's | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating	AA-
Munich Reinsurance America Inc. | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	6,307
Reinsurance Recoverables Paid/Unpaid Losses	70,676
Reinsurance Recoverables Total	76,983	[1]
Collateral Held	4,440	[2]
Munich Reinsurance America Inc. | AM Best | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating	A+
Munich Reinsurance America Inc. | Standard & Poor's | AM Best, A+ Rating | Standard & Poor's, AA- Rating
Ceded Credit Risk [Line Items]
Rating	AA-
Partner Reinsurance Europe | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	11,141
Reinsurance Recoverables Paid/Unpaid Losses	40,623
Reinsurance Recoverables Total	51,764	[1]
Collateral Held	18,715	[2]
Partner Reinsurance Europe | AM Best | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Partner Reinsurance Europe | Standard & Poor's | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Lloyd Syndicate #2003 | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	8,493
Reinsurance Recoverables Paid/Unpaid Losses	37,427
Reinsurance Recoverables Total	45,920	[1]
Collateral Held	10,568	[2]
Lloyd Syndicate #2003 | AM Best | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A
Lloyd Syndicate #2003 | Standard & Poor's | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Allied World Reinsurance | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	14,006
Reinsurance Recoverables Paid/Unpaid Losses	28,718
Reinsurance Recoverables Total	42,724	[1]
Collateral Held	4,724	[2]
Allied World Reinsurance | AM Best | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating	A
Allied World Reinsurance | Standard & Poor's | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating	A
Scor Global P&C SE | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	12,212
Reinsurance Recoverables Paid/Unpaid Losses	25,127
Reinsurance Recoverables Total	37,339	[1]
Collateral Held	11,826	[2]
Scor Global P&C SE | AM Best | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A
Scor Global P&C SE | Standard & Poor's | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Tower Insurance Company | AM Best, B++ Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	13,271
Reinsurance Recoverables Paid/Unpaid Losses	16,002
Reinsurance Recoverables Total	29,273	[1]
Collateral Held	8,305	[2]
Tower Insurance Company | AM Best | AM Best, B++ Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating	B++
Tower Insurance Company | Standard & Poor's | AM Best, B++ Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating	NR
Validus Reinsurance Ltd | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	2,576
Reinsurance Recoverables Paid/Unpaid Losses	21,140
Reinsurance Recoverables Total	23,716	[1]
Collateral Held	12,756	[2]
Validus Reinsurance Ltd | AM Best | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating	A
Validus Reinsurance Ltd | Standard & Poor's | AM Best, A Rating | Standard & Poor's, A Rating
Ceded Credit Risk [Line Items]
Rating	A
Employers Mutual Casualty Company | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	11,257
Reinsurance Recoverables Paid/Unpaid Losses	6,746
Reinsurance Recoverables Total	18,003	[1]
Collateral Held	11,050	[2]
Employers Mutual Casualty Company | AM Best | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating	A
Employers Mutual Casualty Company | Standard & Poor's | AM Best, A Rating | Standard & Poor's, Not Rated
Ceded Credit Risk [Line Items]
Rating	NR
Atlantic Specialty Insurance | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	8,626
Reinsurance Recoverables Paid/Unpaid Losses	8,406
Reinsurance Recoverables Total	17,032	[1]
Collateral Held	2,560	[2]
Atlantic Specialty Insurance | AM Best | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Rating	A
Atlantic Specialty Insurance | Standard & Poor's | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Rating	A-
QBE Reinsurance Corp | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	6,959
Reinsurance Recoverables Paid/Unpaid Losses	9,559
Reinsurance Recoverables Total	16,518	[1]
Collateral Held	2,416	[2]
QBE Reinsurance Corp | AM Best | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A
QBE Reinsurance Corp | Standard & Poor's | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Scor Holding (Switzerland) AG | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	675
Reinsurance Recoverables Paid/Unpaid Losses	14,050
Reinsurance Recoverables Total	14,725	[1]
Collateral Held	4,927	[2]
Scor Holding (Switzerland) AG | AM Best | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A
Scor Holding (Switzerland) AG | Standard & Poor's | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Berkley Insurance Company | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	1,787
Reinsurance Recoverables Paid/Unpaid Losses	12,797
Reinsurance Recoverables Total	14,584	[1]
Collateral Held	519	[2]
Berkley Insurance Company | AM Best | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Berkley Insurance Company | Standard & Poor's | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Lloyd Syndicate #4000 | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	345
Reinsurance Recoverables Paid/Unpaid Losses	13,930
Reinsurance Recoverables Total	14,275	[1]
Collateral Held	305	[2]
Lloyd Syndicate #4000 | AM Best | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A
Lloyd Syndicate #4000 | Standard & Poor's | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
AXIS Re Europe | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	4,750
Reinsurance Recoverables Paid/Unpaid Losses	9,511
Reinsurance Recoverables Total	14,261	[1]
Collateral Held	4,486	[2]
AXIS Re Europe | AM Best | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
AXIS Re Europe | Standard & Poor's | AM Best, A+ Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Lloyd's Syndicate #1955 | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	2,459
Reinsurance Recoverables Paid/Unpaid Losses	10,931
Reinsurance Recoverables Total	13,390	[1]
Collateral Held	2,397	[2]
Lloyd's Syndicate #1955 | AM Best | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A
Lloyd's Syndicate #1955 | Standard & Poor's | AM Best, A Rating | Standard & Poor's, A+ Rating
Ceded Credit Risk [Line Items]
Rating	A+
Platinum Underwriters Re | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	259
Reinsurance Recoverables Paid/Unpaid Losses	12,700
Reinsurance Recoverables Total	12,959	[1]
Collateral Held	1,692	[2]
Platinum Underwriters Re | AM Best | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Rating	A
Platinum Underwriters Re | Standard & Poor's | AM Best, A Rating | Standard & Poor's, A- Rating
Ceded Credit Risk [Line Items]
Rating	A-
Insurance
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	211,424
Reinsurance Recoverables Paid/Unpaid Losses	624,204
Reinsurance Recoverables Total	835,628	[1]
Collateral Held	166,876	[2]
All Other
Ceded Credit Risk [Line Items]
Reinsurance Recoverables Unearned Premium	42,011
Reinsurance Recoverables Paid/Unpaid Losses	217,040
Reinsurance Recoverables Total	259,051	[1]
Collateral Held	$ 73,700	[2]

[1] Net of reserve for uncollectible reinsurance of approximately $11.3 million.

[2] Collateral of $240.6 million consists of $180.6 million in ceded balances payable, $57.2 million in letters of credit, and $2.8 million of other balances held by the Company's Insurance Companies and Lloyd's Operations.