Fair Value Measurements (Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Sep. 30, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Marketable securities	$ 0.8	$ 8.2
Investment in Post Holdings, Inc.	223.1
Derivative assets	1.5	0.3
Deferred compensation investment	25.0	22.9
Total assets	250.4	31.4
Derivative liabilities	24.7	53.1
Deferred compensation liabilities	30.3	36.5
Total liabilities	55.0	89.6
Level 1 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Marketable securities	0.8	8.2
Investment in Post Holdings, Inc.	223.1
Deferred compensation investment	25.0	22.9
Total assets	248.9	31.1
Level 2 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative assets	1.5	0.3
Total assets	1.5	0.3
Derivative liabilities	24.7	53.1
Deferred compensation liabilities	30.3	36.5
Total liabilities	$ 55.0	$ 89.6