Reinsurance Recoverables (Detail) (USD $)	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Ceded Credit Risk [Line Items]
Reinsurance recoverables on paid losses	$ 71,700,000		$ 61,600,000
Ceded outstanding loss and LAE	1,289,400,000	[1]	1,302,100,000	[1]	1,305,900,000	[1]	831,800,000	[1]
Total reinsurance recoverables	$ 1,361,083,000	[2],[3]	$ 1,363,707,000

[1] Reinsurance recoverables in this table include only ceded loss and LAE reserves.

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

[3] 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.