Information Regarding Concentration of Reinsurance Recoverables and The Ratings Profile of Its Reinsurers (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Ceded Credit Risk [Line Items]
Reinsurance Recoverable		$ 1,746,488	$ 1,272,219
Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[1],[2]	$ 1,746,500
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]	$ 525,700
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	30.10%
Reinsurance Recoverable | Reinsurer Concentration Risk | Kane SAC Ltd
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[3],[4]	$ 75,500
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	4.30%
Reinsurance Recoverable | Reinsurer Concentration Risk | All other reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2]	$ 761,200
Reinsurance recoverable as percentage of total reinsurance recoverables	[2]	43.60%
A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk | Swiss Reinsurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 154,200
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	8.80%
A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk | Renaissance Re Holdings Limited
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 97,300
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	5.60%
A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk | W.R. Berkley Corporation
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 88,200
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	5.10%
A+ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk | Hannover Ruck SE
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 55,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	3.20%
A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk | Syndicates at Lloyd's of London
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 133,800
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	7.70%
A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk | PartnerRe Limited
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 119,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	6.80%
A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk | Fairfax Financial Holdings Limited
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4],[5]	$ 101,500
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4],[5]	5.80%
A (Excellent) | Reinsurance Recoverable | Reinsurer Concentration Risk | Liberty Mutual
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 74,000
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	4.20%
A++ (Superior) | Reinsurance Recoverable | Reinsurer Concentration Risk | Chubb Corporation
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	[2],[4]	$ 86,000
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	4.90%

[1] Approximately 78 percent of Alleghany's reinsurance recoverables balance as of December 31, 2017 was due from reinsurers having an A.M. Best Company, Inc. financial strength rating of A (Excellent) or higher.

[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.

[5] In July 2017, Fairfax Financial Holdings Ltd acquired Allied World Assurance Company Holdings, AG.