Information Regarding Concentration of Reinsurance Recoverables and The Ratings Profile of Our Reinsurers (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses		$ 2,167,400	$ 2,199,200
Allowance for credit losses		(2,700)	(3,200)	$ (6,000)	$ (7,900)
Reinsurance recoverables		2,164,653	$ 2,195,975
Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[1],[2]	2,167,400
Allowance for credit losses	[2]	(2,700)
Reinsurance recoverables	[2]	$ 2,164,700
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]	$ 156,800
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	7.20%
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]	$ 131,600
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	6.10%
PartnerRe Limited | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 161,800
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	7.50%
RenaissanceRe Holdings Ltd | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 128,100
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	5.90%
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]	$ 128,900
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	5.90%
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]	4.60%
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]	$ 86,700
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	4.00%
Chubb Ltd. | A++ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 80,100
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	3.70%
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]	$ 76,100
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],[4]	$ 70,900
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	3.30%
All other reinsurers | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2]	$ 1,047,200
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]	$ 957,600
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	44.20%

[1] Approximately 66 percent of Alleghany's reinsurance recoverables balance as of March 31, 2022 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.