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Share-Based Payments - Incentive Plans (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2019
Stock Incentive Plans |  |  |
Employer contributions, compensation expense | $ 26.1 | $ 33.7 | $ 32.3
Employee Stock Option |  |  |
Stock Incentive Plans |  |  |
Securities available for future issuance (in shares) | 12.1 |  |
Expiration term of unexercised options | 7 years |  |
Number of Options |  |  |
Balance at the beginning of the period (in shares) | 0.4 | 0.1 | 0.6
Granted (in shares) |  | 0.3 |
Exercised (in shares) | (0.1) |  |
Cancelled (in shares) |  |  | (0.5)
Balance at the end of the period (in shares) | 0.3 | 0.4 | 0.1
Exercisable at the end of the period (in shares) |  | 0.1 | 0.1
Stock options, Weighted average exercise price |  |  |
Balance at the beginning of the period (in dollars per share) | $ 36.41 | $ 31.62 | $ 31.62
Granted (in dollars per share) |  | 38.72 |
Exercised (in dollars per share) | 31.62 |  |
Cancelled (in dollars per share) |  |  | (31.62)
Balance at the end of the period (in dollars per share) | $ 38.72 | 36.41 | 31.62
Exercisable at the end of the period (in dollars per share) |  | $ 31.62 | $ 31.62

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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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