Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Sep. 30, 2011
Marketable securities, current	$ 0.8	$ 8.2
Post stock, cost basis	245.0
Net losses in accumulated other comprehensive income of marketable securities	21.9
Fixed rate debt, carrying amount	1,940.9	1,951.6
Fixed rate debt, estimated fair value	2,247.2	2,070.1
Post Common Stock [Member]
Marketable securities, current	$ 223.1