Redeemable Noncontrolling Interest (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, beginning of period	$ 2,021.6
Redeemable noncontrolling interest, end of period	2,997.3	$ 2,021.6
Net earnings available to Class A common stockholders	27.6	23.5	$ 0.0
BellRing Brands, Inc.
Redeemable Noncontrolling Interest [Line Items]
Impact of initial public offering	0.0	1,364.6
Redemption value adjustment to noncontrolling interest	887.5	591.9
Net earnings available to Class A common stockholders	27.6	23.5
Change from net earnings available to class A common stockholders and effects of changes, net	$ 915.1	$ 1,980.0
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	71.20%	71.20%
BellRing Brands, LLC unit | BellRing Brands, Inc.
Redeemable Noncontrolling Interest [Line Items]
Noncontrolling interest, ownership percentage by parent	28.80%	28.80%
Noncontrolling interest
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, beginning of period	$ 2,021.6	$ 0.0
Net earnings attributable to redeemable noncontrolling interest subsequent to IPO	86.8	71.1
Net change in hedges, net of tax	1.6	(6.7)
Foreign currency translation adjustments	(0.2)	0.7
Impact of initial public offering	0.0	1,364.6
Redemption value adjustment to noncontrolling interest	887.5	591.9
Redeemable noncontrolling interest, end of period	$ 2,997.3	$ 2,021.6	$ 0.0