Information Regarding Concentration of Reinsurance Recoverables and Ratings Profile of its Reinsurers (Detail) (USD $)				9 Months Ended
	Sep. 30, 2012		Dec. 31, 2011	Sep. 30, 2012 Swiss Reinsurance American Corporation		Sep. 30, 2012 American International Group, Inc.		Sep. 30, 2012 Platinum Underwriters Holdings, Ltd.		Sep. 30, 2012 PartnerRe Ltd.		Sep. 30, 2012 Syndicates at Lloyd's of London		Sep. 30, 2012 All other reinsurers
Ceded Credit Risk [Line Items]
Rating				A+(Superior)	[1],[2]	A (Excellent)	[1],[2]	A (Excellent)	[1],[2]	A+(Superior)	[1],[2]	A (Excellent)	[1],[2]
Reinsurance Recoverable	$ 1,271,555,000	[1]	$ 852,845,000	$ 162,200,000	[1]	$ 107,600,000	[1]	$ 95,700,000	[1]	$ 90,800,000	[1]	$ 75,300,000	[1]	$ 740,000,000	[1],[3]
Secured reinsurance recoverables	$ 179,900,000	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	100.00%	[1]		12.80%	[1]	8.50%	[1]	7.50%	[1]	7.10%	[1]	5.90%	[1]	58.20%	[1],[3]
Secured reinsurance recoverables, percentage	14.10%	[1],[4]

[1] Reinsurance recoverables reflect amounts due from one or more reinsurance subsidiaries of the listed company.

[2] Represents the A.M. Best Company financial strength rating for the applicable reinsurance subsidiary or subsidiaries from which the reinsurance recoverable is due.

[3] Approximately 92.1 percent of Alleghany's reinsurance recoverables balance as of September 30, 2012 was due from reinsurers having an A.M. Best Company financial strength rating of "A" (Excellent) or higher.

[4] Represents reinsurance recoverables secured by funds held, trust agreements and letters of credit.