Net earnings attributable to redeemable noncontrolling interest subsequent to IPO

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Change in noncontrolling interest during the period as a result of a change in the redemption value of redeemable noncontrolling interest.

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The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

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The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

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Amount after tax, before reclassification adjustments of gain (loss) on foreign currency translation adjustments, foreign currency transactions designated and effective as economic hedges of a net investment in a foreign entity and intra-entity foreign currency transactions that are of a long-term-investment nature.

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Amount after tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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As of the reporting date, the aggregate carrying amount of all noncontrolling interests which are redeemable by the (parent) entity (1) at a fixed or determinable price on a fixed or determinable date, (2) at the option of the holder of the noncontrolling interest, or (3) upon occurrence of an event that is not solely within the control of the (parent) entity. This item includes noncontrolling interest holder's ownership (or holders' ownership) regardless of the type of equity interest (common, preferred, other) including all potential organizational (legal) forms of the investee entity.

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Impact on total stockholders' equity of cumulative effect of change in reporting entity. A change in reporting entity is considered as follows: (1) presenting consolidated statements in place of the financial statements of individual reporting entities; (2) changing the combination of subsidiaries that make up the group for which the consolidated financial statements are presented; and (3) changing the entities included in the combined financial statements. A business combination accounted for under the purchase method and the consolidation of a variable interest entity are not considered a change in reporting entity.

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Redeemable Noncontrolling Interest (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Oct. 21, 2019
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, beginning of period				$ 0.0					$ 0.0
Net earnings attributable to redeemable noncontrolling interest subsequent to IPO									71.1
Redeemable noncontrolling interest, end of period	$ 2,021.6				$ 0.0				2,021.6	$ 0.0
Net earnings available to Class A common stockholders	$ 10.0	$ 3.3	$ 4.2	6.0	$ 0.0	$ 0.0	$ 0.0	$ 0.0	23.5	$ 0.0	$ 0.0
BellRing Brands, Inc.
Redeemable Noncontrolling Interest [Line Items]
Impact of initial public offering									1,364.6
Redemption value adjustment to noncontrolling interest									591.9
Net earnings available to Class A common stockholders									23.5
Change from net earnings available to class A common stockholders and effects of changes, net									$ 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%				100.00%				71.20%	100.00%		71.20%
BellRing Brands, LLC unit \| BellRing Brands, Inc.
Redeemable Noncontrolling Interest [Line Items]
Noncontrolling interest, ownership percentage by parent	28.80%								28.80%			28.80%
Noncontrolling interest
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, beginning of period				$ 0.0					$ 0.0
Net earnings attributable to redeemable noncontrolling interest subsequent to IPO									71.1
Net change in hedges, net of tax									(6.7)
Foreign currency translation adjustments									0.7
Impact of initial public offering									1,364.6
Redemption value adjustment to noncontrolling interest									591.9
Redeemable noncontrolling interest, end of period	$ 2,021.6				$ 0.0				$ 2,021.6	$ 0.0