Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Goodwill [Line Items]
Goodwill (gross)	$ 3,777.1	$ 3,770.2	$ 3,527.1
Accumulated impairment losses	(697.4)	(697.4)	(640.4)
Goodwill	3,079.7	3,072.8	2,886.7
Goodwill acquired	$ 4.2	263.8
Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount	36.00%
Impairment Loss		(57.0)	(212.6)
Purchase Accounting Adjustments		5.2
Currency translation adjustment	$ 2.7	(25.9)
Post Consumer Brands Segment
Goodwill [Line Items]
Goodwill (gross)	1,994.0	1,993.9	1,799.3
Accumulated impairment losses	(609.1)	(609.1)	(609.1)
Goodwill	1,384.9	1,384.8	1,190.2
Goodwill acquired	0.0	195.6
Impairment Loss			(181.3)
Purchase Accounting Adjustments		0.0
Currency translation adjustment	0.1	(1.0)
Michael Foods Group Segment
Goodwill [Line Items]
Goodwill (gross)	1,345.8	1,341.6	1,347.2
Accumulated impairment losses	0.0	0.0	0.0
Goodwill	1,345.8	1,341.6	1,347.2
Goodwill acquired	4.2	0.0
Impairment Loss		0.0
Purchase Accounting Adjustments		(5.6)
Currency translation adjustment	0.0	0.0
Active Nutrition Segment
Goodwill [Line Items]
Goodwill (gross)	180.7	180.7	151.3
Accumulated impairment losses	(88.3)	(88.3)	(31.3)
Goodwill	92.4	92.4	120.0
Goodwill acquired	0.0	18.6
Impairment Loss		(57.0)	(31.3)
Purchase Accounting Adjustments		10.8
Currency translation adjustment	0.0	0.0
Private Brands Segment
Goodwill [Line Items]
Goodwill (gross)	256.6	254.0	229.3
Accumulated impairment losses	0.0	0.0	0.0
Goodwill	256.6	254.0	$ 229.3
Goodwill acquired	0.0	49.6
Impairment Loss		0.0
Purchase Accounting Adjustments		0.0
Currency translation adjustment	2.6	$ (24.9)
Dymatize
Goodwill [Line Items]
Reporting Unit, Amount of Fair Value in Excess of Carrying Amount	$ 36.0
All reporting units excluding Dymatize [Member]
Goodwill [Line Items]
Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount	33.00%