Document and Entity Information In Millions, unless otherwise specified	9 Months Ended
	Aug. 31, 2014	Sep. 30, 2014
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Aug. 31, 2014
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	Q3
Trading Symbol	GY
Entity Registrant Name	GENCORP INC
Entity Central Index Key	0000040888
Current Fiscal Year End Date	--11-30
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		58.8