Recently Issued Accounting Standards (Details) (USD $)	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2009
Balance Sheet Classification [Line Items]
New accounting pronouncement or change in accounting principle effect of adoption quantification	$ 247,100,000	$ 137,800,000
Investment in Ralcorp Receivables Corporation	0	137,800,000	134,400,000
Short-Term Debt [Member]
Balance Sheet Classification [Line Items]
New accounting pronouncement or change in accounting principle effect of adoption quantification	$ 105,000,000	$ 0