Stock-Based Compensation - Narrative (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Post Long-Term Incentive Plans
Share-based Payment Award [Line Items]
Maximum award vesting period	10 years
Share-based payment arrangement, expense	$ 1.0	$ 2.6	$ 3.9
Share-based payment arrangement, expense, tax benefit	$ 0.1	0.2	0.3
BellRing 2019 Long-Term Incentive Plan
Share-based Payment Award [Line Items]
Maximum award vesting period	10 years
Share-based payment arrangement, expense	$ 9.8	4.6	2.5
Share-based payment arrangement, expense, tax benefit	1.2	$ 0.3	$ 0.2
Share-based payment arrangement, nonvested award, cost not yet recognized, amount	$ 20.8
Share-based payment arrangement, nonvested award, cost not yet recognized, period for recognition	2 years
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	1.7

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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- Definition

Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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Amount of tax benefit for recognition of expense of award under share-based payment arrangement.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

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-Publisher FASB
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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