Stock-Based Compensation - Narrative (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Post Long-Term Incentive Plans
Share-based Payment Award [Line Items]
Maximum award vesting period	10 years
Share-based payment arrangement, expense	$ 2.6	$ 3.9	$ 3.5
Share-based payment arrangement, expense, tax benefit	0.2	0.3	$ 0.8
Share-based payment arrangement, nonvested award, cost not yet recognized, amount	$ 1.0
Share-based payment arrangement, nonvested award, cost not yet recognized, period for recognition	8 months 12 days
BellRing 2019 Long-Term Incentive Plan
Share-based Payment Award [Line Items]
Maximum award vesting period	10 years
Share-based payment arrangement, expense	$ 4.6	2.5
Share-based payment arrangement, expense, tax benefit	0.3	$ 0.2
Share-based payment arrangement, nonvested award, cost not yet recognized, amount	$ 6.9
Share-based payment arrangement, nonvested award, cost not yet recognized, period for recognition	1 year 7 months 6 days
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	1.9

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

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

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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.

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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.

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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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