Discontinued Operations (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Percentage of pro rata share distribution from business unit Spin-Off	80.30%		80.30%
Discontinued operation, period of continuing cash flows after disposal, years				two
Net Cash Provided by Financing Activities - Discontinued Operations			$ 900.0
Other operating expenses from discontinued operations	10.1	0.7	13.5	3.5
Post Shareholder [Member]
Stockholders' equity note, stock split, conversion ratio			1
Percentage of retained pro rata share distribution from business unit spinoff	19.70%		19.70%
Ralcorp Shareholder [Member]
Stockholders' equity note, stock split, conversion ratio			2
Post [Member]
Threshold upon which common stock of entity will be disposed of			1
Amount due to Post included in accounts payable	7.6		7.6
Transition Services Agreement [Member] | Post [Member]
Other operating expenses from discontinued operations	$ 2.2