Guarantor Financials - Consolidated Statements of Cash Flows (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2010
Condensed Financial Statements, Captions      
Proceeds from issuance of Senior Notes $ 775.0 $ 0 $ 0
Proceeds from Issuance of Secured Debt 175.0 0 0
Repayments of Related Party Debt 900.0 0 0
Repayments of long-term debt (4.4) 0 0
Payments for Repurchase of Common Stock 53.4 0 0
Payments of Debt Issuance Costs 17.7 0 0
Cash Flows from Operating Activities:      
Net Cash Provided (Used) by Operating Activities 144.0 143.8 135.6
Cash Flows from Investing Activities:      
Additions to property and intangible assets 30.9 14.9 24.3
Payments for Equity Contributions 0    
Proceeds from Equity Distributions 0    
Net cash used by investing activities (30.9) (14.9) (24.3)
Cash Flows from Financing Activities:      
Change in net investment of Ralcorp (39.4) (192.3) (112.4)
Proceeds from Equity Contributions 0    
Payments for Equity Distributions 0    
Changes in intercompany debt 7.8 60.2 0
Net cash used by financing activities (57.1) (132.1) (112.4)
Effect of exchange rate changes on cash 0.5 0.1 0.2
Net Decrease in Cash and Cash Equivalents 56.5 (3.1) (0.9)
Cash and cash equivalents, beginning of year 1.7 4.8 5.7
Cash and cash equivalents, end of year 58.2 1.7 4.8
Parent Company
     
Condensed Financial Statements, Captions      
Proceeds from issuance of Senior Notes 775.0    
Proceeds from Issuance of Secured Debt 175.0    
Repayments of Related Party Debt (900.0)    
Repayments of long-term debt (4.4)    
Payments for Repurchase of Common Stock (53.4)    
Payments of Debt Issuance Costs (17.7)    
Cash Flows from Operating Activities:      
Net Cash Provided (Used) by Operating Activities (3.1) 0 0
Cash Flows from Investing Activities:      
Additions to property and intangible assets 0 0 0
Payments for Equity Contributions (6.0)    
Proceeds from Equity Distributions 84.3    
Net cash used by investing activities 78.3 0 0
Cash Flows from Financing Activities:      
Change in net investment of Ralcorp 0 0 0
Proceeds from Equity Contributions 0    
Payments for Equity Distributions 0    
Changes in intercompany debt 0 0  
Net cash used by financing activities (25.5) 0 0
Effect of exchange rate changes on cash 0 0 0
Net Decrease in Cash and Cash Equivalents 49.7 0 0
Cash and cash equivalents, beginning of year 0 0 0
Cash and cash equivalents, end of year 49.7 0 0
Guarantors
     
Condensed Financial Statements, Captions      
Proceeds from issuance of Senior Notes 0    
Proceeds from Issuance of Secured Debt 0    
Repayments of Related Party Debt 0    
Repayments of long-term debt 0    
Payments for Repurchase of Common Stock 0    
Payments of Debt Issuance Costs 0    
Cash Flows from Operating Activities:      
Net Cash Provided (Used) by Operating Activities 201.5 144.6 132.7
Cash Flows from Investing Activities:      
Additions to property and intangible assets (29.5) 12.5 19.9
Payments for Equity Contributions 0    
Proceeds from Equity Distributions 0    
Net cash used by investing activities (29.5) (12.5) (19.9)
Cash Flows from Financing Activities:      
Change in net investment of Ralcorp 0 (132.1) (112.8)
Proceeds from Equity Contributions 0    
Payments for Equity Distributions (148.5)    
Changes in intercompany debt (21.3) 0  
Net cash used by financing activities (169.8) (132.1) (112.8)
Effect of exchange rate changes on cash 0 0 0
Net Decrease in Cash and Cash Equivalents 2.2 0 0
Cash and cash equivalents, beginning of year 0 0 0
Cash and cash equivalents, end of year 2.2 0 0
Non-Guarantors
     
Condensed Financial Statements, Captions      
Proceeds from issuance of Senior Notes 0    
Proceeds from Issuance of Secured Debt 0    
Repayments of Related Party Debt 0    
Repayments of long-term debt 0    
Payments for Repurchase of Common Stock 0    
Payments of Debt Issuance Costs 0    
Cash Flows from Operating Activities:      
Net Cash Provided (Used) by Operating Activities 9.8 (0.8) 2.9
Cash Flows from Investing Activities:      
Additions to property and intangible assets (1.4) 2.4 4.4
Payments for Equity Contributions 0    
Proceeds from Equity Distributions 0    
Net cash used by investing activities (1.4) (2.4) (4.4)
Cash Flows from Financing Activities:      
Change in net investment of Ralcorp 7.8 (60.2) 0.4
Proceeds from Equity Contributions 6.0    
Payments for Equity Distributions 0    
Changes in intercompany debt (18.1) 60.2  
Net cash used by financing activities (4.3) 0 0.4
Effect of exchange rate changes on cash 0.5 0.1 0.2
Net Decrease in Cash and Cash Equivalents 4.6 (3.1) (0.9)
Cash and cash equivalents, beginning of year 1.7 4.8 5.7
Cash and cash equivalents, end of year 6.3 1.7 4.8
Eliminations
     
Condensed Financial Statements, Captions      
Proceeds from issuance of Senior Notes 0    
Proceeds from Issuance of Secured Debt 0    
Repayments of Related Party Debt 0    
Repayments of long-term debt 0    
Payments for Repurchase of Common Stock 0    
Payments of Debt Issuance Costs 0    
Cash Flows from Operating Activities:      
Net Cash Provided (Used) by Operating Activities (64.2) 0 0
Cash Flows from Investing Activities:      
Additions to property and intangible assets 0 0 0
Payments for Equity Contributions 6.0    
Proceeds from Equity Distributions (84.3)    
Net cash used by investing activities (78.3) 0 0
Cash Flows from Financing Activities:      
Change in net investment of Ralcorp 0 0 0
Proceeds from Equity Contributions (6.0)    
Payments for Equity Distributions 148.5    
Changes in intercompany debt 0 0  
Net cash used by financing activities 142.5 0 0
Effect of exchange rate changes on cash 0 0 0
Net Decrease in Cash and Cash Equivalents 0 0 0
Cash and cash equivalents, beginning of year 0 0 0
Cash and cash equivalents, end of year $ 0 $ 0 $ 0