Redeemable Noncontrolling Interest (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Sep. 30, 2022	Mar. 10, 2022	Mar. 09, 2022
Redeemable Noncontrolling Interest [Line Items]
Net earnings available to common stockholders	$ 44.2	$ 8.2
Redeemable noncontrolling interest, end of period	$ 0.0
BellRing Brands, Inc.
Redeemable Noncontrolling Interest [Line Items]
Net earnings available to common stockholders		8.2
Redemption value adjustment to redeemable noncontrolling interest		(247.6)
Changes from net earnings available to common stockholders and transfers from NCI		(239.4)
Redeemable noncontrolling interest
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, beginning of period		2,997.3
Net earnings attributable to NCI		31.1
Net change in hedges, net of tax		0.4
Foreign currency translation adjustments		(0.3)
Redemption value adjustment to redeemable noncontrolling interest		(247.6)
Redeemable noncontrolling interest, end of period		$ 2,780.9
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
Noncontrolling interest, ownership percentage by noncontrolling owners			3.40%
BellRing Common Stock Ownership Percentage				14.20%