Information Regarding Concentration of Reinsurance Recoverables and Ratings Profile of its Reinsurers (Detail) (USD $)				3 Months Ended
	Mar. 31, 2012		Dec. 31, 2011	Mar. 31, 2012 Swiss Reinsurance American Corporation		Mar. 31, 2012 Chartis, a subsidiary of AIG		Mar. 31, 2012 Platinum Underwriters Holdings, Ltd.		Mar. 31, 2012 PartnerRe Ltd.		Mar. 31, 2012 Syndicates at Lloyd's		Mar. 31, 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	$ 1,324,420,000	[1]	$ 852,845,000	$ 166,600,000	[1]	$ 109,000,000	[1]	$ 97,100,000	[1]	$ 89,600,000	[1]	$ 87,300,000	[1]	$ 774,800,000	[1],[3]
Secured reinsurance recoverables	$ 191,100,000	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	100.00%	[1]		12.60%	[1]	8.20%	[1]	7.30%	[1]	6.80%	[1]	6.60%	[1]	58.50%	[1],[3]
Secured reinsurance recoverables, Percentage	14.40%	[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 rating for the applicable reinsurance subsidiary or subsidiaries from which the reinsurance recoverable is due.

[3] Approximately 86.8 percent of Alleghany's reinsurance recoverables balance as of March 31, 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.