Segment Information (Summary Of Operating Segments) (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	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information [Line Items]
Net Sales	$ 1,062.2	$ 917.3	$ 2,228.7	$ 1,869.0
Total segment operating profit	70.4	103.5	173.4	197.4
Interest expense, net	(32.0)	(33.8)	(66.4)	(69.5)
Adjustments for economic hedges	(2.8)	4.9	(8.0)	8.8
Merger and integration costs	(1.8)	(0.1)	(7.4)	(0.3)
Amortization of intangible assets	(20.1)	(16.3)	(40.9)	(32.6)
Provision for legal settlement	(4.4)		(4.4)	(2.5)
Amounts related to plant closures	(6.7)	(0.2)	(6.8)	(0.4)
Stock-based compensation expense	(5.5)	(3.4)	(8.2)	(6.7)
Systems upgrade and conversion costs	(1.6)	(1.5)	(3.2)	(3.9)
Other unallocated corporate expenses	(10.9)	(10.3)	(22.0)	(21.5)
Earnings from Continuing Operations before Income Taxes	38.4	69.7	107.0	127.9
Depreciation and Amortization	48.8	41.7	98.2	83.9
Cash and cash equivalents	131.4	41.4	131.4	41.4	50.0	29.3
Assets of discontinued operations					2,672.0
Investment in Post Holdings, Inc.	223.1		223.1
Other unallocated corporate assets	298.7		298.7		318.9
Total Assets	4,452.7		4,452.7		6,279.2
Cereal Products [Member]
Segment Reporting Information [Line Items]
Net Sales	219.9	198.9	446.1	403.6
Total segment operating profit	20.3	22.5	49.1	45.0
Depreciation and Amortization	5.0	5.4	10.3	10.9
Total Assets	261.4		261.4		263.4
Snacks, Sauces & Spreads [Member]
Segment Reporting Information [Line Items]
Net Sales	407.7	382.2	881.3	799.6
Total segment operating profit	32.8	33.3	73.9	70.7
Depreciation and Amortization	10.8	10.2	21.4	20.5
Total Assets	807.5		807.5		799.0
Accelerated Amortization [Member]
Segment Reporting Information [Line Items]
Amortization of intangible assets	(1.2)	(1.2)	(2.5)	(2.5)
Frozen Bakery Products [Member]
Segment Reporting Information [Line Items]
Net Sales	275.3	192.7	583.3	386.4
Total segment operating profit	26.7	22.9	60.7	45.9
Depreciation and Amortization	17.3	9.7	34.2	19.7
Total Assets	1,266.1		1,266.1		712.9
Pasta [Member]
Segment Reporting Information [Line Items]
Net Sales	159.3	143.5	318.0	279.4
Total segment operating profit	25.5	36.6	52.2	64.8
Depreciation and Amortization	13.0	13.1	25.7	26.3
Total Assets	1,464.5		1,464.5		1,463.0
Corporate [Member]
Segment Reporting Information [Line Items]
Depreciation and Amortization	2.7	3.3	6.6	6.5
Total Segment [Member]
Segment Reporting Information [Line Items]
Total segment operating profit	105.3	115.3	235.9	226.4
Total Assets	$ 3,799.5		$ 3,799.5		$ 3,238.3