Summary Of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended					3 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2009	Sep. 30, 2011 Machinery And Equipment [Member]	Sep. 30, 2011 Building and Building Improvements [Member]	Sep. 30, 2011 Post Honey Bunches Of Oats Post Selects And Post Trademarks [Member]	Jun. 30, 2011 Post Shredded Wheat And Grape-Nuts Trademarks [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives range, maximum, years				15	30
Depreciation expense	$ 148.3	$ 117.5	$ 102.4
Amortization expense, fiscal year 2012	79.3
Amortization expense, fiscal year 2013	70.9
Amortization expense, fiscal year 2014	66.1
Amortization expense, fiscal year 2015	61.3
Amortization expense, fiscal year 2016	56.2
Impairment loss						106.6	32.1
Storage and other warehouse cost	$ 144.7	$ 127.6	$ 111.6