Derivative Financial Instruments and Hedging Gain(Loss) recognized in earnings from derivative instruments (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2010
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative Instruments, Gain (Loss) Recognized in Income, Net $ (2.0) $ (13.6) $ (0.9)
Participation in Ralcorp derivative program costs [Member] | Cost of Sales [Member]
     
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative Instruments, Gain (Loss) Recognized in Income, Net (2.0) (13.6) (0.9)
Commodity Contract [Member] | Cost of Sales [Member]
     
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative Instruments, Gain (Loss) Recognized in Income, Net 0 0 0
Future [Member] | Cost of Sales [Member]
     
Derivative Instruments, Gain (Loss) [Line Items]      
Derivative Instruments, Gain (Loss) Recognized in Income, Net $ 0.3 $ 0 $ 0