Segments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Oct. 01, 2018
Segment Reporting Information [Line Items]
Investment, Ownership Percentage, Parent												60.50%
Net Sales	$ 1,442.8	$ 1,439.2	$ 1,387.8	$ 1,411.3	$ 1,629.9	$ 1,608.1	$ 1,586.1	$ 1,433.1	$ 5,681.1	$ 6,257.2	$ 5,225.8
Entity-Wide Revenue, Major Customer, Percentage									21.00%	18.00%	17.00%
Operating Profit									$ 781.0	$ 573.5	$ 516.7
Gain on sale of business	0.7	0.0	(2.6)	(124.7)					(126.6)	0.0	0.0
Impairment of goodwill and other intangible assets	63.3	$ 0.0	$ 0.0	$ 0.0	124.9	$ 0.0	$ 0.0	$ 0.0	63.3	124.9	26.5
Interest expense, net									322.4	387.3	314.8
Loss on extinguishment of debt, net									6.1	31.1	222.9
Expense (income) on swaps, net									306.6	(95.6)	(91.8)
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest									159.1	264.7	74.4
Property, Plant and Equipment, Additions									273.9	225.0	190.4
Depreciation and amortization									379.6	398.4	323.1
Assets	11,951.6				13,057.5				11,951.6	13,057.5	11,876.8
Major customer sales, value
Segment Reporting Information [Line Items]
Net Sales									$ 1,186.8	$ 1,100.6	$ 914.2
Non-US
Segment Reporting Information [Line Items]
Sales to Foreign Countries as a Percentage of Total Net Sales									13.00%	13.00%	8.00%
Disclosure on Geographic Areas, Long-Lived Assets in Foreign Countries	279.0				284.3				$ 279.0	$ 284.3
Operating Segments
Segment Reporting Information [Line Items]
Net Sales									5,681.1	6,257.2	$ 5,225.8
Operating Profit									900.5	849.2	634.5
Depreciation and amortization									363.6	390.2	319.4
Operating Segments | Post Consumer Brands Segment
Segment Reporting Information [Line Items]
Net Sales									1,875.9	1,831.7	1,742.5
Operating Profit									337.1	329.2	354.9
Property, Plant and Equipment, Additions									62.1	51.5	57.8
Depreciation and amortization									117.4	122.0	112.4
Assets	3,296.3				3,391.7				3,296.3	3,391.7	3,440.5
Operating Segments | Weetabix Segment
Segment Reporting Information [Line Items]
Net Sales									418.2	423.4	112.4
Operating Profit									94.8	87.2	14.5
Property, Plant and Equipment, Additions									37.7	26.3	13.6
Depreciation and amortization									35.0	38.1	7.7
Assets	1,779.1				1,853.3				1,779.1	1,853.3	2,048.9
Operating Segments | Foodservice and Refrigerated Retail
Segment Reporting Information [Line Items]
Property, Plant and Equipment, Additions									162.3	114.6	66.0
Assets	5,033.8				5,132.4				5,033.8	5,132.4	3,176.0
Operating Segments | Foodservice Segment
Segment Reporting Information [Line Items]
Net Sales									1,627.4	1,548.2	1,340.6
Operating Profit									198.4	157.6	26.9
Depreciation and amortization									111.8	105.4	98.6
Operating Segments | Refrigerated Retail Segment
Segment Reporting Information [Line Items]
Net Sales									907.3	790.9	530.2
Operating Profit									95.1	90.0	83.7
Depreciation and amortization									74.1	57.9	26.8
Operating Segments | Active Nutrition segment
Segment Reporting Information [Line Items]
Net Sales									854.4	827.5	713.2
Operating Profit									175.1	124.4	96.4
Property, Plant and Equipment, Additions									3.2	5.0	3.9
Depreciation and amortization									25.3	25.9	25.3
Assets	594.0				559.3				594.0	559.3	581.3
Operating Segments | Private Brands segment
Segment Reporting Information [Line Items]
Net Sales									0.0	848.9	791.2
Operating Profit									0.0	60.8	58.1
Property, Plant and Equipment, Additions									0.0	26.6	29.1
Depreciation and amortization									0.0	40.9	48.6
Assets	0.0				1,055.3				0.0	1,055.3	1,054.9
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Other Expenses									169.6	136.8	87.7
Property, Plant and Equipment, Additions									8.6	1.0	20.0
Depreciation and amortization									16.0	8.2	3.7
Assets	$ 1,248.4				1,065.5				1,248.4	1,065.5	1,575.2
Intersegment Eliminations
Segment Reporting Information [Line Items]
Net Sales									(2.1)	(13.4)	(4.3)
Cereal and granola | Operating Segments
Segment Reporting Information [Line Items]
Net Sales									2,293.3	2,351.2	1,963.9
Eggs and egg products | Operating Segments
Segment Reporting Information [Line Items]
Net Sales									1,578.4	1,542.8	1,419.1
Side Dishes | Operating Segments
Segment Reporting Information [Line Items]
Net Sales									519.6	398.2	192.3
Cheese and dairy | Operating Segments
Segment Reporting Information [Line Items]
Net Sales									234.6	248.6	259.4
Sausage | Operating Segments
Segment Reporting Information [Line Items]
Net Sales									149.6	96.0	0.0
Protein-based products and supplements | Operating Segments
Segment Reporting Information [Line Items]
Net Sales									854.7	827.5	713.2
Nut butters and dried fruit and nut | Operating Segments
Segment Reporting Information [Line Items]
Net Sales									0.0	487.5	432.5
Pasta | Operating Segments
Segment Reporting Information [Line Items]
Net Sales									0.0	258.4	249.4
Other | Operating Segments
Segment Reporting Information [Line Items]
Net Sales									52.5	53.0	0.0
Product Eliminations
Segment Reporting Information [Line Items]
Net Sales									$ (1.6)	(6.0)	$ (4.0)
Assets Held For Sale | Non-US
Segment Reporting Information [Line Items]
Disclosure on Geographic Areas, Long-Lived Assets in Foreign Countries					$ 12.4					$ 12.4