Redeemable Noncontrolling Interest (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020	Mar. 10, 2022	Mar. 09, 2022
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, beginning of period	$ 2,997.3
Redeemable noncontrolling interest, end of period	0.0	$ 2,997.3
Net earnings available to common stockholders	82.3	27.6	$ 23.5
BellRing Brands, Inc.
Redeemable Noncontrolling Interest [Line Items]
Redemption value adjustment to noncontrolling interest	(370.5)	887.5	591.9
Increase in equity as a result of the Spin-off	(2,665.1)	0.0	0.0
Impact of initial public offering	0.0	0.0	1,364.6
Net earnings available to common stockholders	82.3	27.6	23.5
Change from net earnings available to common stockholders and effects of changes, net	$ (2,953.3)	$ 915.1	$ 1,980.0
BellRing Brands, LLC unit | Post Holdings, Inc.
Redeemable Noncontrolling Interest [Line Items]
Common units, outstanding		97.5	97.5	19.4	97.5
Noncontrolling interest, ownership percentage by noncontrolling owners		71.20%	71.20%		71.50%
BellRing Common Stock | Post Holdings, Inc.
Redeemable Noncontrolling Interest [Line Items]
Noncontrolling interest, ownership percentage by noncontrolling owners	3.40%
BellRing Common Stock Ownership Percentage				14.20%
BellRing Common Stock | Old BellRing Stockholders
Redeemable Noncontrolling Interest [Line Items]
BellRing Common Stock Ownership Percentage		28.80%	28.80%	28.50%
Noncontrolling interest
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, beginning of period	$ 2,997.3	$ 2,021.6	$ 0.0
Net earnings attributable to redeemable noncontrolling interest subsequent to IPO	33.7	86.8	71.1
Net change in hedges, net of tax	5.1	1.6	(6.7)
Foreign currency translation adjustments	(0.5)	(0.2)	0.7
Impact of IPO	0.0	0.0	1,364.6
Redemption value adjustment to noncontrolling interest	(370.5)	887.5	591.9
Impact of Spin-off	(2,665.1)	0.0	0.0
Redeemable noncontrolling interest, end of period	$ 0.0	$ 2,997.3	$ 2,021.6