Information Regarding Concentration of Reinsurance Recoverables and Ratings Profile of its Reinsurers (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	$ 1,361,083	[1],[2]	$ 1,363,707
Reinsurance recoverable as percentage of total reinsurance recoverables	100.00%	[1],[2]
Ceded Credit Risk, Secured
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	161,700	[1],[3]
Reinsurance recoverable as percentage of total reinsurance recoverables	11.90%	[1],[3]
All other reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	480,300	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	35.20%	[1],[4]
A+ (Superior) | Swiss Reinsurance Company
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	150,500	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	11.10%	[1],[4]
A+ (Superior) | PartnerRe Limited
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	110,300	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	8.10%	[1],[4]
A+ (Superior) | W.R. Berkley Corporation
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	88,600	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	6.50%	[1],[4]
A+ (Superior) | Allianz SE
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	39,300	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	2.90%	[1],[4]
A+ (Superior) | Munich Reinsurance
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	38,300	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	2.80%	[1],[4]
A (Excellent) | American International Group Incorporated
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	143,900	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	10.60%	[1],[4]
A (Excellent) | Syndicates at Lloyd's of London
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	98,300	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	7.20%	[1],[4]
A (Excellent) | Platinum Underwriters Holdings Limited
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	93,900	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	6.90%	[1],[4]
A++ (Superior) | Chubb Corporation
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	64,900	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	4.80%	[1],[4]
A++ (Superior) | Ace Ltd
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	$ 52,800	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	3.90%	[1],[4]

[1] Reinsurance recoverables reflect amounts due from one or more reinsurance subsidiaries of the listed company.

[2] Approximately 96 percent of Alleghany's reinsurance recoverables balance as of December 31, 2014 was due from reinsurers having an A.M. Best Company, Inc. financial strength rating of A (Excellent) or higher.

[3] Represents reinsurance recoverables secured by funds held, trust agreements and 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.