Segment Information (Operating Segments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2009	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2009	Sep. 30, 2008
Segment Reporting Information [Line Items]
Net sales	$ 1,223.2	$ 1,171.9	$ 1,172.6	$ 1,173.3	$ 1,129.2	$ 962.4	$ 965.0	$ 991.9	$ 4,741.0	$ 4,048.5	$ 3,891.9
Segment operating profit									29.8	421.9	448.3
Interest expense, net									(134.0)	(107.8)	(99.0)
Adjustments for economic hedges	(18.5)	(21.2)	6.0	4.8					(28.9)
Merger and integration costs	(1.0)	(1.2)	(0.1)	(0.2)	(14.5)	(13.5)	(4.5)	(0.6)	(2.5)	(33.1)	(32.0)
Post separation costs	(2.8)								(2.8)
Impairment of intangible assets	(471.4)	(32.1)			(19.4)		(20.5)		(503.5)	(39.9)
Provision for legal settlement				(2.5)	(7.5)				(2.5)	(7.5)
Amounts related to plant closures	(1.3)	(2.4)	(0.2)	(0.2)	(1.5)	(0.2)	(0.1)	(0.7)	(4.1)	(2.5)	(0.5)
Amortization of intangible assets									78.2	49.3	42.3
Gain on forward sale contracts											17.6
Gain on sale of securities											70.6
Stock-based compensation expense									(16.4)	(17.9)	(13.4)
Systems upgrade and conversion costs									(7.7)	(9.6)	(0.5)
Other unallocated corporate expenses									(38.7)	(33.5)	(34.6)
(Loss) earnings before income taxes and equity earnings									(104.2)	314.1	437.5
Additions to property and intangibles									141.1	128.9	115.0
Depreciation and amortization									226.5	166.8	144.7
Cash and Cash Equivalents	50.0				29.3				50.0	29.3	282.8	14.1
Investment in Ralcorp Receivables Corporation	0				137.8				0	137.8	134.4
Other unallocated corporate assets	374.3				137.8				374.3	137.8	85.9
Assets	6,333.2				6,804.9				6,333.2	6,804.9	5,452.2
Branded Cereal Products [Member]
Segment Reporting Information [Line Items]
Net sales									953.8	987.5	1,070.6
Segment operating profit									206.0	220.6	250.6
Additions to property and intangibles									14.7	24.3	36.8
Depreciation and amortization									58.7	55.4	50.6
Assets	2,670.6				3,271.3				2,670.6	3,271.3	3,351.7
Other Cereal Products [Member]
Segment Reporting Information [Line Items]
Net sales									838.5	799.7	803.3
Segment operating profit									86.3	90.3	92.0
Additions to property and intangibles									14.5	15.6	13.8
Depreciation and amortization									21.2	21.3	20.7
Assets	263.4				268.7				263.4	268.7	269.5
Snacks, Sauces & Spreads [Member]
Segment Reporting Information [Line Items]
Net sales									1,602.7	1,461.6	1,323.2
Segment operating profit									135.5	152.6	117.6
Additions to property and intangibles									58.7	50.9	29.6
Depreciation and amortization									41.0	36.2	31.1
Assets	799.0				760.0				799.0	760.0	604.0
Frozen Bakery Products [Member]
Segment Reporting Information [Line Items]
Net sales									768.6	698.3	694.8
Segment operating profit									88.0	80.8	69.1
Additions to property and intangibles									21.3	23.7	12.3
Depreciation and amortization									39.4	36.7	35.4
Assets	712.9				743.4				712.9	743.4	723.9
Pasta [Member]
Segment Reporting Information [Line Items]
Net sales									577.4	101.4
Segment operating profit									126.1	21.6
Additions to property and intangibles									20.9	2.8
Depreciation and amortization									52.2	8.7
Assets	1,463.0				1,456.6				1,463.0	1,456.6
Corporate [Member]
Segment Reporting Information [Line Items]
Additions to property and intangibles									11.0	11.6	22.5
Depreciation and amortization									14.0	8.5	6.9
Segment Total [Member]
Segment Reporting Information [Line Items]
Segment operating profit									641.9	565.9	529.3
Assets	5,908.9				6,500.0				5,908.9	6,500.0	4,949.1
Accelerated Amortization [Member]
Segment Reporting Information [Line Items]
Amortization of intangible assets									$ (5.0)