Stock-Based Compensation Plans - Restricted Stock Units (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Post Stock Settled
Share-based Payment Award [Line Items]
Nonvestsed restricted stock awards	929,933	1,013,939
Granted	351,786
Vested	(413,820)
Forfeited	(21,972)
Grant date value per share	$ 89.14	$ 81.27	$ 71.94
Granted, Weighted Average Exercise Price	103.97
Vested, Weighted Average Exercise Price	82.16
Forfeited, Weighted Average Exercise Price	$ 95.08
Fair value of restricted stock awards vested	$ 42.1	$ 24.9	$ 17.4
Post Cash Settled
Share-based Payment Award [Line Items]
Grant date price of restricted stock units valued at greater of current stock price or grant price	$ 51.43
Nonvestsed restricted stock awards	39,200	49,000
Granted	0
Vested	(9,800)
Forfeited	0
Grant date value per share	$ 51.43	$ 51.43
Granted, Weighted Average Exercise Price	0
Vested, Weighted Average Exercise Price	51.43
Forfeited, Weighted Average Exercise Price	$ 0
Total Share-based Liabilities Paid	$ 0.9	$ 1.1	$ 3.2
BOD Grant
Share-based Payment Award [Line Items]
Granted			13,300
Share-based Compensation Arrangement by Share-based Payment Award, Accelerated Vesting, Number			11,400
Non-employee director share-based compensation			$ 0.9

Grant date price of restricted stock units valued at greater of current stock price or grant price

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Non-employee director share-based compensation

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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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Amount of cash paid to settle liability for award under share-based payment arrangement.

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

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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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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Number of shares for which recognition of cost was accelerated for award under share-based payment arrangement.

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