Redeemable Noncontrolling Interest (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Sep. 30, 2022	Mar. 10, 2022	Mar. 09, 2022
Redeemable Noncontrolling Interest [Line Items]
Net earnings available to common stockholders	$ 44.3	$ 39.1	$ 119.4	$ 48.6
Redeemable noncontrolling interest, end of period	$ 0.0		$ 0.0
BellRing Brands, Inc.
Redeemable Noncontrolling Interest [Line Items]
Redemption value adjustment to redeemable noncontrolling interest				(370.5)
Net earnings available to common stockholders				48.6
Increase in equity as a result of the Spin-off				(2,665.1)
Changes from net earnings available to common stockholders and transfers from NCI				(2,987.0)
Redeemable noncontrolling interest
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, beginning of period				2,997.3
Net earnings attributable to NCI				33.7
Net change in hedges, net of tax				5.1
Foreign currency translation adjustments				(0.5)
Redemption value adjustment to redeemable noncontrolling interest				(370.5)
Noncontrolling Interest, Impact of Spin-off				(2,665.1)
Redeemable noncontrolling interest, end of period		$ 0.0		$ 0.0
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%
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%