Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Sep. 30, 2021
Segment Reporting Information
Net Sales	$ 1,524.9	$ 1,247.5	$ 4,272.1	$ 3,624.8
Interest expense, net	75.6	82.4	245.6	249.7
(Gain) loss on extinguishment of debt, net	(10.2)	0.0	9.1	93.2
(Income) expense on swaps, net	(131.6)	121.6	(222.9)	(105.6)
Earnings (loss) before income taxes and equity method loss	219.6	(46.1)	748.7	185.5
Depreciation and amortization	93.8	91.7	285.5	268.6
Assets	11,560.2		11,560.2		$ 12,414.7
Gain on investment in BellRing	(35.1)	0.0	(482.8)	0.0
Disposal Group, Including Discontinued Operation, Assets	0.0		0.0		694.1
Continuing Operations
Segment Reporting Information
Assets					11,720.6
5.75% Senior Notes Maturing March 2027 | Loss on extinguishment of debt and refinancing fees, net | Senior Notes
Segment Reporting Information
(Gain) loss on extinguishment of debt, net			15.8
Corporate
Segment Reporting Information
General corporate expenses and other	47.6	0.7	138.7	29.6
Depreciation and amortization	0.8	1.1	2.7	3.1
Assets	1,247.4		1,247.4		1,248.2
Operating Segments
Segment Reporting Information
Net Sales	1,524.9	1,247.5	4,272.1	3,624.8
Segment Profit (Loss)	165.9	158.6	436.4	452.4
Depreciation and amortization	93.0	90.6	282.8	265.5
Operating Segments | Post Consumer Brands
Segment Reporting Information
Net Sales	574.7	468.7	1,655.1	1,393.6
Segment Profit (Loss)	81.8	87.8	232.6	250.1
Depreciation and amortization	33.3	30.0	101.0	87.4
Assets	3,532.1		3,532.1		3,467.8
Operating Segments | Weetabix
Segment Reporting Information
Net Sales	124.9	123.4	360.5	350.3
Segment Profit (Loss)	27.8	28.6	81.8	82.6
Depreciation and amortization	8.9	9.8	27.8	28.6
Assets	1,769.6		1,769.6		1,930.4
Operating Segments | Foodservice
Segment Reporting Information
Net Sales	579.0	435.1	1,469.5	1,158.8
Segment Profit (Loss)	45.9	27.9	81.0	47.5
Depreciation and amortization	31.8	32.0	95.3	94.3
Operating Segments | Refrigerated Retail
Segment Reporting Information
Net Sales	246.4	220.8	787.4	723.4
Segment Profit (Loss)	10.4	14.3	41.0	72.2
Depreciation and amortization	19.0	18.8	58.7	55.2
Operating Segments | Foodservice and Refrigerated Retail
Segment Reporting Information
Assets	5,011.1		5,011.1		$ 5,074.2
Eliminations | Segment Eliminations
Segment Reporting Information
Net Sales	(0.1)	(0.5)	(0.4)	(1.3)
Cereal and granola | Operating Segments
Segment Reporting Information
Net Sales	657.8	570.1	1,936.5	1,704.1
Nut butters | Operating Segments
Segment Reporting Information
Net Sales	35.5	21.8	72.5	39.2
Eggs and egg products | Operating Segments
Segment Reporting Information
Net Sales	560.6	416.1	1,420.7	1,108.0
Side dishes (including potato products) | Operating Segments
Segment Reporting Information
Net Sales	158.2	133.7	489.3	427.6
Cheese and dairy | Operating Segments
Segment Reporting Information
Net Sales	51.0	50.8	164.6	168.1
Sausage | Operating Segments
Segment Reporting Information
Net Sales	38.8	37.4	131.2	124.7
Other | Operating Segments
Segment Reporting Information
Net Sales	16.9	18.4	51.9	54.6
Product Eliminations | Eliminations
Segment Reporting Information
Net Sales	(0.1)	(0.8)	(0.8)	(1.5)
Protein-based shakes | Operating Segments
Segment Reporting Information
Net Sales	$ 6.2	$ 0.0	$ 6.2	$ 0.0