Stock-Based Compensation - Performance-based RSUs (Details) - $ / shares	12 Months Ended
Sep. 30, 2023	Sep. 30, 2022
Minimum
Share-based Payment Award [Line Items]
Total Award Range	0.00%
Maximum
Share-based Payment Award [Line Items]
Total Award Range	260.00%
BRBR Performance-Based Restricted Stock Units
Share-based Payment Award [Line Items]
RSUs nonvested, beginning balance	375,219
RSUs granted in period	176,062
RSUs vested in period	0
RSUs forfeited in period	0
RSUs nonvested, ending balance	551,281	375,219
RSUs nonvested, weighted average grant date fair value	$ 42.66	$ 41.44
RSUs granted in period, weighted average grant date fair value	45.26
RSUs vested in period, weighted average grant date fair value	0
RSUs forfeited in period, weighted average grant date fair value	$ 0
Fair Value Assumptions, expected term	3 years	2 years 10 months 24 days
Fair Value Assumptions, expected volatility rate	46.80%	49.60%
Fair Value Assumptions, risk free interest rate	4.10%	2.30%
Fair Value Assumptions, weighted average grant date fair value	$ 45.26	$ 42.33
Stock options, expected dividend rate	0.00%	0.00%

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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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 weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Expected term of 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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