Information Regarding Concentration of Reinsurance Recoverables and Ratings Profile of its Reinsurers (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	$ 1,363,707		$ 1,348,599
Reinsurance recoverable as percentage of total reinsurance recoverables	100.00%	[1],[2]
Ceded Credit Risk, Secured
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	152,000	[1],[3]
Reinsurance recoverable as percentage of total reinsurance recoverables	11.00%	[1],[3]
Swiss Reinsurance Company | A+ (Superior)
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	157,600	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	11.40%	[1],[4]
American International Group, Inc. | A (Excellent)
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	132,600	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	9.60%	[1],[4]
PartnerRe Ltd. | A+ (Superior)
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	107,100	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	7.70%	[1],[4]
Platinum Underwriters Holdings, Ltd. | A (Excellent)
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	94,900	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	6.90%	[1],[4]
Syndicates at Lloyd's of London | A (Excellent)
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	93,200	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	6.70%	[1],[4]
W.R. Berkley Corporation | A+ (Superior)
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	79,200	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	5.70%	[1],[4]
Chubb Corporation | A+ (Superior)
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	73,400	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	5.30%	[1],[4]
Ace Ltd | A+ (Superior)
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	53,800	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	3.90%	[1],[4]
XL Group | A (Excellent)
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	50,700	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	3.70%	[1],[4]
Munich Reinsurance | A+ (Superior)
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	42,800	[1],[4]
Reinsurance recoverable as percentage of total reinsurance recoverables	3.10%	[1],[4]
All other reinsurers
Ceded Credit Risk [Line Items]
Reinsurance Recoverable	$ 478,400	[1]
Reinsurance recoverable as percentage of total reinsurance recoverables	36.00%	[1]

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

[2] Approximately 94.8 percent of Alleghany's reinsurance recoverables balance as of December 31, 2013 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.