Redeemable Noncontrolling Interest (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Mar. 10, 2022	Mar. 09, 2022
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, end of period	$ 0.0
Net earnings available to common stockholders	165.5	$ 82.3	$ 27.6
BellRing Brands, Inc.
Redeemable Noncontrolling Interest [Line Items]
Redemption value adjustment to noncontrolling interest		(370.5)	887.5
Increase in equity as a result of the Spin-off		(2,665.1)	0.0
Net earnings available to common stockholders		82.3	27.6
Change from net earnings available to common stockholders and effects of changes, net		$ (2,953.3)	915.1
BellRing Common Stock | Post Holdings, Inc.
Redeemable Noncontrolling Interest [Line Items]
Common units, outstanding				19.4
BellRing Common Stock Ownership Percentage		3.40%		14.20%
BellRing Brands, LLC unit | Post Holdings, Inc.
Redeemable Noncontrolling Interest [Line Items]
Common units, outstanding					97.5
Noncontrolling interest, ownership percentage by noncontrolling owners					71.50%
BellRing Brands, LLC unit | BellRing Brands, Inc.
Redeemable Noncontrolling Interest [Line Items]
Noncontrolling interest, ownership percentage by parent					28.50%
Noncontrolling interest
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, beginning of period	$ 0.0	$ 2,997.3	2,021.6
Net earnings attributable to redeemable noncontrolling interest subsequent to IPO		33.7	86.8
Net change in hedges, net of tax		5.1	1.6
Foreign currency translation adjustments		(0.5)	(0.2)
Redemption value adjustment to noncontrolling interest		(370.5)	887.5
Impact of Spin-off		(2,665.1)	0.0
Redeemable noncontrolling interest, end of period		$ 0.0	$ 2,997.3