Stock-Based Compensation - Stock Options (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
PHI Stock Options
Share-based Payment Award [Line Items]
Stock options outstanding, beginning balance	38,314
Stock options granted in period	0
Stock options exercised in period	0
Stock options forfeited in period	0
Stock options expired in period	0
Stock options outstanding, ending balance	38,314	38,314
Stock options vested and expected to vest	38,314
Stock options exercisable	24,763
Stock options outstanding, weighted average exercise price	$ 81.42	$ 81.42
Stock options granted in period, weighted average exercise price	0
Stock options exercised in period, weighted average exercise price	0
Stock options forfeited in period, weighted average exercise price	0
Stock options expired in period, weighted average exercise price	0
Stock options vested and expected to vest, weighted average exercise price	81.42
Stock options exercisable in period, weighted average exercise price	$ 78.29
Stock options outstanding, weighted average remaining contractual term	7 years 2 months 15 days
Stock options vested and expected to vest, weighted average remaining contractual term	7 years 2 months 15 days
Stock options exercisable, weighted average remaining contractual term	6 years 10 months 28 days
Stock options outstanding, intrinsic value	$ 0.3
Stock options vested and expected to vest, intrinsic value	0.3
Stock options exercisable, intrinsic value	$ 0.2
Stock options, expected term		6 years 6 months	6 years 6 months
Stock options, expected volatility rate		29.70%	30.70%
Stock options, risk free interest rate		3.10%	2.30%
Stock options, expected dividend rate		0.00%	0.00%
Stock options, weighted average grant date fair value		$ 33.82	$ 28.42
BRBR Stock Options
Share-based Payment Award [Line Items]
Stock options outstanding, beginning balance	0
Stock options granted in period	96,000
Stock options exercised in period	0
Stock options forfeited in period	0
Stock options expired in period	0
Stock options outstanding, ending balance	96,000	0
Stock options vested and expected to vest	96,000
Stock options exercisable	0
Stock options outstanding, weighted average exercise price	$ 19.31	$ 0
Stock options granted in period, weighted average exercise price	19.31
Stock options exercised in period, weighted average exercise price	0
Stock options forfeited in period, weighted average exercise price	0
Stock options expired in period, weighted average exercise price	0
Stock options vested and expected to vest, weighted average exercise price	$ 19.31
Stock options outstanding, weighted average remaining contractual term	9 years 1 month 24 days
Stock options vested and expected to vest, weighted average remaining contractual term	9 years 1 month 24 days
Stock options outstanding, intrinsic value	$ 0.1
Stock options vested and expected to vest, intrinsic value	$ 0.1
Stock options, expected term	6 years 6 months
Stock options, expected volatility rate	38.50%
Stock options, risk free interest rate	1.60%
Stock options, expected dividend rate	0.00%
Stock options, weighted average grant date fair value	$ 7.92

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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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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- Definition

Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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- Definition

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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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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- Definition

Number of options outstanding, including both vested and non-vested options.

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- Definition

Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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- Definition

Weighted average price at which option holders acquired shares when converting their stock options into shares.

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- Definition

Weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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- Definition

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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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding.

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The weighted average price as of the balance sheet date at which grantees could acquire the underlying shares with respect to all outstanding stock options which are in the customized range of exercise prices.

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Number of share options (or share units) exercised during the current period.

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