Redeemable Noncontrolling Interest (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Mar. 10, 2022	Mar. 09, 2022	Sep. 30, 2021
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, beginning of period			$ 2,997.3
Net earnings available to common stockholders	$ 39.1	$ 9.5	48.6	$ 17.9
Redeemable noncontrolling interest, end of period	0.0		0.0
Level 1
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, beginning of period			2,997.3
BellRing Brands, Inc.
Redeemable Noncontrolling Interest [Line Items]
Redemption value adjustment to redeemable noncontrolling interest	0.0	723.9	(370.5)	975.9
Net earnings available to common stockholders	39.1	9.5	48.6	17.9
Increase in equity as a result of the Spin-off	0.0	0.0	(2,665.1)	0.0
Changes from net earnings available to common stockholders and transfers to (from) NCI	$ 39.1	733.4	$ (2,987.0)	993.8
BellRing Brands, LLC unit | Post Holdings, Inc.
Redeemable Noncontrolling Interest [Line Items]
Common units, outstanding						97.5	97.5
Noncontrolling interest, ownership percentage by noncontrolling owners						71.50%	71.20%
BellRing Brands, LLC unit | BellRing Brands, Inc.
Redeemable Noncontrolling Interest [Line Items]
Noncontrolling interest, ownership percentage by parent						28.50%	28.80%
BellRing Common Stock | Post Holdings, Inc.
Redeemable Noncontrolling Interest [Line Items]
BellRing Common Stock Ownership Percentage	14.20%		14.20%		14.20%
Redeemable noncontrolling interest
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, beginning of period	$ 0.0	2,301.4	$ 2,997.3	2,021.6
Net earnings attributable to NCI	0.0	29.0	33.7	56.0
Net change in hedges, net of tax	0.0	0.4	5.1	1.2
Foreign currency translation adjustments	0.0	0.2	(0.5)	0.2
Redemption value adjustment to redeemable noncontrolling interest	0.0	723.9	(370.5)	975.9
Noncontrolling Interest, Impact of Spin-off	0.0	0.0	(2,665.1)	0.0
Redeemable noncontrolling interest, end of period	$ 0.0	$ 3,054.9	$ 0.0	$ 3,054.9