Information Regarding Concentration of Reinsurance Recoverables and The Ratings Profile of Its Reinsurers (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Ceded Credit Risk [Line Items]
Reinsurance Recoverable		$ 1,921,278	$ 1,746,488
Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[1],[2]	$ 1,921,300
Reinsurance recoverable as percentage of total reinsurance recoverables	[1],[2]	100.00%
Reinsurance Recoverable | Reinsurer Concentration Risk | Ceded Credit Risk, Secured
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[3]	$ 711,100
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	37.00%
Reinsurance Recoverable | Reinsurer Concentration Risk | All other reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2]	$ 914,000
Reinsurance recoverable as percentage of total reinsurance recoverables	[2]	47.60%
A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk | Swiss Reinsurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 112,900
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	5.90%
A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk | Renaissance Re Holdings Limited
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 103,900
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	5.40%
A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk | W.R. Berkley Corporation
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 89,600
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	4.70%
A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk | Syndicates at Lloyd's of London
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 134,500
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	7.00%
A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk | PartnerRe Limited
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 114,200
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	5.90%
A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk | Fairfax Financial Holdings Limited
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 97,300
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	5.10%
A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk | Liberty Mutual
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 85,000
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	4.40%
A++ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk | Chubb Corporation
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 81,200
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	4.20%
Not Rated | Reinsurance Recoverable | Reinsurer Concentration Risk | Kane SAC Ltd Bowery Segregated Account [Member]
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[3],[4]	$ 88,900
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	4.60%
Not Rated | Reinsurance Recoverable | Reinsurer Concentration Risk | Kane SAC Ltd Rondout Segregated Account [Member]
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[3],[4]	$ 99,800
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	5.20%

[1] Approximately 70 percent of Alleghany’s reinsurance recoverables balance as of December 31, 2018 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 reinsurance recoverables secured by funds held, trust agreements or letters of credit.

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