Information Regarding Concentration of Reinsurance Recoverables and The Ratings Profile of Our Reinsurers (Detail) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses		$ 2,167,300		$ 2,199,200
Allowance for credit losses		(5,900)	$ (2,700)	(3,200)	$ (2,600)	$ (6,000)	$ (7,900)
Reinsurance recoverables		2,161,436		$ 2,195,975
Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[1],[2]	2,167,300
Allowance for credit losses	[2]	(5,900)
Reinsurance recoverables	[2]	$ 2,161,400
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]	$ 160,200
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	7.40%
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]	$ 137,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	6.30%
PartnerRe Limited | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 176,200
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	8.10%
RenaissanceRe Holdings Ltd | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 130,300
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	6.00%
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]	$ 136,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	6.30%
Swiss Reinsurance Company | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 99,100
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]	$ 89,600
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	4.10%
Chubb Ltd. | A++ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 84,800
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	3.90%
Stone Ridge Holding Group | Not Rated | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3],[4]	$ 78,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	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]	$ 78,000
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	3.60%
All other reinsurers | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2]	$ 996,900
Reinsurance recoverable as percentage of total reinsurance recoverables	[2]	46.10%
Ceded Credit Risk, Secured | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables	[2],[4]	$ 940,300
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	43.40%

[1] Approximately 68 percent of Alleghany’s reinsurance recoverables balance as of June 30, 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.