Information Regarding Concentration of Reinsurance Recoverables and The Ratings Profile of Our Reinsurers (Parenthetical) (Detail) - Reinsurance Recoverable - Reinsurer Concentration Risk	12 Months Ended
	Dec. 31, 2020
Ceded Credit Risk [Line Items]
Reinsurance recoverable as percentage of total reinsurance recoverables	100.00%	[1],[2]
Reinsurers Rated A Or Higher
Ceded Credit Risk [Line Items]
Reinsurance recoverable as percentage of total reinsurance recoverables	71.00%

[1] Approximately 71 percent of Alleghany’s reinsurance recoverables balance as of December 31, 2020 was due from reinsurers having an A.M. Best Company, Inc. financial strength rating of A (Excellent) or higher, with a majority of the other reinsurance recoverables being secured by funds held, trust agreements or letters of credit.

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