Fair Value Measurements (Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Sep. 30, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Marketable securities	$ 4.7	$ 8.2
Investment in Post Holdings, Inc.	208.3
Derivative assets		0.3
Deferred compensation investment	22.8	22.9
Total assets	235.8	31.4
Derivative liabilities	13.3	53.1
Deferred compensation liabilities	28.0	36.5
Total liabilities	41.3	89.6
Level 1 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Marketable securities	4.7	8.2
Investment in Post Holdings, Inc.	208.3
Deferred compensation investment	22.8	22.9
Total assets	235.8	31.1
Level 2 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative assets		0.3
Total assets		0.3
Derivative liabilities	13.3	53.1
Deferred compensation liabilities	28.0	36.5
Total liabilities	41.3	89.6
Parent Company [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment in Post Holdings, Inc.	$ 208.3