Goodwill and Intangible Asset, Net of Accumulated Amortization Expense on Consolidated Balance Sheets (Detail) (USD $)	Sep. 30, 2012		Dec. 31, 2011
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill and other intangible assets, Gross carrying value	$ 515,900,000		$ 193,300,000	[1]
Finite lived intangible assets, accumulated amortization	267,700,000		54,300,000	[1]
Goodwill and other intangible assets, Net carrying value	248,200,000	[2]	139,000,000	[1],[2]
Goodwill	82,495,000		48,095,000
Intangible assets excluding goodwill, net of amortization	165,669,000		90,863,000
Insurance Segment
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill and other intangible assets, Gross carrying value	193,300,000		193,300,000	[1]
Finite lived intangible assets, accumulated amortization	56,900,000		54,300,000	[1]
Goodwill and other intangible assets, Net carrying value	136,400,000	[2]	139,000,000	[1],[2]
Goodwill, gross	48,100,000	[3]	48,100,000	[1],[3]
Goodwill	48,100,000	[2],[3]	48,100,000	[1],[2],[3]
Intangible assets excluding goodwill, gross	145,200,000		145,200,000	[1]
Intangible assets excluding goodwill, net of amortization	88,300,000	[2]	90,900,000	[1],[2]
Insurance Segment | Agency Relationships
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, accumulated amortization	7,800,000		7,100,000	[1]
Finite lived intangible assets, gross	21,700,000		21,700,000	[1]
Finite lived intangible assets, net	13,900,000	[2]	14,600,000	[1],[2]
Insurance Segment | Brokerage and Reinsurance Relationships
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, accumulated amortization	20,800,000		19,100,000	[1]
Finite lived intangible assets, gross	33,800,000		33,800,000	[1]
Finite lived intangible assets, net	13,000,000	[2]	14,700,000	[1],[2]
Insurance Segment | Renewal and distribution rights
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, accumulated amortization	24,200,000		24,000,000	[1]
Finite lived intangible assets, gross	24,300,000		24,300,000	[1]
Finite lived intangible assets, net	100,000	[2]	300,000	[1],[2]
Insurance Segment | Other
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, accumulated amortization	4,100,000		4,100,000	[1]
Finite lived intangible assets, gross	4,100,000		4,100,000	[1]
Insurance Segment | State insurance licenses
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite lived intangible assets, gross	25,800,000		25,800,000	[1]
Indefinite lived intangible assets, net	25,800,000	[2]	25,800,000	[1],[2]
Insurance Segment | Trade name
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite lived intangible assets, gross	35,500,000		35,500,000	[1]
Indefinite lived intangible assets, net	35,500,000	[2]	35,500,000	[1],[2]
Corporate activities
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill and other intangible assets, Gross carrying value	35,100,000
Goodwill and other intangible assets, Net carrying value	35,100,000	[2]
Goodwill, gross	34,400,000	[3],[4]
Goodwill	34,400,000	[2],[3],[4]
Intangible assets excluding goodwill, gross	700,000	[4]
Intangible assets excluding goodwill, net of amortization	700,000	[2],[4]
Corporate activities | Other
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	300,000
Finite lived intangible assets, net	300,000	[2]
Corporate activities | Trade name
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite lived intangible assets, gross	400,000
Indefinite lived intangible assets, net	400,000	[2]
Reinsurance Segment
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, accumulated amortization	210,800,000	[3]
Intangible assets excluding goodwill, gross	287,500,000	[3]
Intangible assets excluding goodwill, net of amortization	76,700,000	[2],[3]
Reinsurance Segment | Value of business in-force
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, accumulated amortization	222,200,000
Finite lived intangible assets, gross	291,400,000
Finite lived intangible assets, net	69,200,000	[2]
Reinsurance Segment | Loss and LAE reserves
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, accumulated amortization	(13,300,000)
Finite lived intangible assets, gross	(98,800,000)
Finite lived intangible assets, net	(85,500,000)	[2]
Reinsurance Segment | Renewal rights
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, accumulated amortization	1,300,000
Finite lived intangible assets, gross	44,000,000
Finite lived intangible assets, net	42,700,000	[2]
Reinsurance Segment | Leases
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, accumulated amortization	(1,600,000)
Finite lived intangible assets, gross	(28,100,000)
Finite lived intangible assets, net	(26,500,000)	[2]
Reinsurance Segment | Gain contingency on dispute previously in arbitration
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, net		[2],[5]		[1],[2],[5]
Reinsurance Segment | Internally-developed software
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, accumulated amortization	2,200,000
Finite lived intangible assets, gross	10,000,000
Finite lived intangible assets, net	7,800,000	[2]
Reinsurance Segment | Trade name
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite lived intangible assets, gross	50,000,000
Indefinite lived intangible assets, net	50,000,000	[2]
Reinsurance Segment | State and foreign insurance licenses
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite lived intangible assets, gross	19,000,000
Indefinite lived intangible assets, net	$ 19,000,000	[2]

[1] See Note 4 of Notes to Consolidated Financial Statements set forth in Item 8 of the 2011 10-K for additional detail on goodwill and other intangible assets.

[2] Goodwill and intangible assets have been reduced by amounts written-down in prior periods. See Note 4(a) of Notes to Consolidated Financial Statements set forth in Item 8 of the 2011 10-K for additional detail on amounts written-down in prior periods.

[3] See Note 13 for additional detail on Alleghany's segments of business.

[4] Represents goodwill and other intangible assets related to the acquisition of Bourn & Koch on April 26, 2012, which was purchased for $55.0 million, including estimated contingent consideration of $8.0 million based on future profitability.

[5] In connection with its accounting for the acquisition of Transatlantic, Alleghany established an asset of $36.0 million, representing an estimate based on the minimum recovery previously agreed to by the parties, net of estimated legal costs. On January 26, 2012, Transatlantic reached an agreement with AIG to settle and mediate a dispute previously in arbitration. On July 20, 2012, in accordance with the agreement between the parties, a mediator awarded Transatlantic a settlement payment of $75.0 million. A pre-tax net benefit of approximately $23.5 million resulting from this settlement was recorded as other income in the third quarter and first nine months of 2012, representing the portion of the settlement payment above the minimum recovery, net of additional estimated legal costs.