Information Regarding Concentration of Reinsurance Recoverables and Ratings Profile of its Reinsurers (Parenthetical) (Detail)	Dec. 31, 2013
Ceded Credit Risk [Line Items]
Percentage reinsurance recoverables	100.00%	[1],[2]
A (Excellent) or higher
Ceded Credit Risk [Line Items]
Percentage reinsurance recoverables	94.80%

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

[2] Approximately 94.8 percent of Alleghany's reinsurance recoverables balance as of December 31, 2013 was due from reinsurers having an A.M. Best Company, Inc. financial strength rating of A (Excellent) or higher.