Information Regarding Concentration of Reinsurance Recoverables and The Ratings Profile of Our Reinsurers (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses		$ 2,199,200	$ 1,789,000
Allowance for credit losses		(3,200)	(7,900)	$ 0
Reinsurance recoverables		2,195,975	$ 1,781,096
Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[1],[2]	2,199,200
Allowance for credit losses	[2]	(3,200)
Reinsurance recoverables	[2]	$ 2,196,000
Reinsurance recoverable as percentage of total reinsurance recoverables	[1],[2]	100.00%
Syndicates at Lloyd's of London | A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 154,500
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	7.00%
Kane SAC Ltd, Rondout Segregated Account | Not Rated | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3],[4]	$ 154,300
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	7.00%
PartnerRe Limited | A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 142,700
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	6.50%
RenaissanceRe Holdings Ltd | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 135,500
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	6.20%
Fairfax Financial Holdings Ltd | A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 124,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	5.70%
Swiss Reinsurance Company | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 99,200
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	4.50%
Integral Reinsurance Ltd | Not Rated | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3],[4]	$ 97,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	4.40%
Chubb Ltd. | A++ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 79,300
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	3.60%
W.R. Berkley Corporation | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 77,000
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	3.50%
SiriusPoint Ltd. | A- (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 73,600
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	3.30%
All other reinsurers | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2]	$ 1,061,300
Reinsurance recoverable as percentage of total reinsurance recoverables	[2]	48.30%
Ceded Credit Risk, Secured | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables	[2],[4]	$ 967,700
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	44.00%

[1] Approximately 66 percent of Alleghany’s reinsurance recoverables balance as of December 31, 2021 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.

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

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