Information Regarding Concentration of Reinsurance Recoverables and The Ratings Profile of Our Reinsurers (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses		$ 1,789,000	$ 1,682,000
Allowance for credit losses		(7,900)	0
Reinsurance recoverables		1,781,096	$ 1,681,962
Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[1],[2]	1,789,000
Allowance for credit losses	[2]	(7,900)
Reinsurance recoverables	[2]	$ 1,781,100
Reinsurance recoverable as percentage of total reinsurance recoverables	[1],[2]	100.00%
Syndicates at Lloyd's of London | A (Excellent/Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 151,100
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	8.40%
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]	$ 124,500
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	7.00%
PartnerRe Limited | A (Excellent/Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 113,100
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	6.30%
Fairfax Financial Holdings Limited | A (Excellent/Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 106,000
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	5.90%
RenaissanceRe Holdings Ltd | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 119,200
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	6.70%
Swiss Reinsurance Company | A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 98,600
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	5.50%
Third Point Reinsurance Group | A- (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 79,100
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	4.40%
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]	$ 73,100
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	4.10%
Chubb | A++ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3]	$ 68,700
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	3.80%
Kane SAC Ltd, Bowery Segregated Account | Not Rated | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3],[4]	$ 62,100
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	3.50%
All other reinsurers | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2]	$ 793,500
Reinsurance recoverable as percentage of total reinsurance recoverables	[2]	44.40%
Ceded Credit Risk, Secured | Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables	[2],[4]	$ 632,100
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	35.50%

[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.

[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.