Segment Information (Operating Segments) (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Sep. 30, 2010	Sep. 30, 2009
Net Sales	$ 1,171.9	$ 962.4	$ 3,517.8	$ 2,919.3
Total segment profit	75.6	104.1	386.8	333.9
Interest (expense) income, net	(33.0)	(24.7)	(102.5)	(75.1)
Provision for legal settlement			(2.5)
Adjustments for economic hedges	(21.2)		(10.4)
Merger and integration costs	(1.2)	(13.5)	(1.5)	(18.6)
Amounts related to plant closures	(2.4)	(0.2)	(2.8)	(1.0)
Impairment of intangible assets	(32.1)		(32.1)	(20.5)
Stock-based compensation expense	(4.5)	(2.4)	(12.4)	(11.9)
Systems upgrade and conversion costs	(1.5)	(1.2)	(5.4)	(3.8)
Earnings before income taxes	42.6	79.4	284.3	258.8
Depreciation and amortization			169.5	116.7
Cash and cash equivalents	47.2	115.3	47.2	115.3	29.3	282.8
Investment in Ralcorp Receivables Corporation					137.8
Total Assets	6,738.1		6,738.1		6,804.9
Parent Company [Member]
Net Sales	129.9	119.0	388.3	382.4
Total segment profit	(23.8)	(12.7)	(9.7)	(14.5)
Interest (expense) income, net	(33.7)	(25.5)	(104.0)	(76.9)
Earnings before income taxes	(57.5)	(38.2)	(113.7)	(91.4)
Cash and cash equivalents	1.0	97.4	1.0	97.4	0.6	263.5
Investment in Ralcorp Receivables Corporation					180.0
Total Assets	5,772.0		5,772.0		5,851.0
Branded Cereal Products [Member]
Net Sales	244.1	242.7	721.0	749.2
Total segment profit	54.0	54.1	160.6	158.2
Depreciation and amortization	14.7	14.2	43.8	41.6
Total Assets	3,174.5		3,174.5		3,271.3
Other Cereal Products [Member]
Net Sales	217.1	197.6	620.7	585.8
Total segment profit	21.9	22.0	64.4	68.2
Depreciation and amortization	6.4	5.3	19.8	15.8
Total Assets	268.3		268.3		268.7
Snacks, Sauces & Spreads [Member]
Net Sales	383.0	359.9	1,182.6	1,067.4
Total segment profit	26.0	35.9	96.7	123.9
Depreciation and amortization	10.0	9.0	30.5	25.9
Total Assets	760.4		760.4		760.0
Frozen Bakery Products [Member]
Net Sales	187.3	162.2	573.7	516.9
Total segment profit	21.5	17.5	67.4	62.1
Depreciation and amortization	10.1	9.1	29.8	26.8
Total Assets	721.5		721.5		743.4
Pasta [Member]
Net Sales	140.4		419.8
Total segment profit	27.1		91.9
Depreciation and amortization	13.1		39.4
Total Assets	1,455.1		1,455.1		1,456.6
Corporate [Member]
Other unallocated corporate expenses	(12.0)	(8.1)	(27.1)	(22.7)
Depreciation and amortization	2.2	1.7	6.2	6.6
Other unallocated corporate assets	311.1		311.1		137.8
Segment Total [Member]
Net Sales	1,171.9	962.4	3,517.8	2,919.3
Total segment profit	150.5	129.5	481.0	412.4
Depreciation and amortization	56.5	39.3	169.5	116.7
Total Assets	$ 6,379.8		$ 6,379.8		$ 6,500.0