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Share-Based Payments - Incentive Plans (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2016
Stock Incentive Plans |  |  |
Compensation expense related to employer contributions to purchase AECOM stock under defined contribution plans, including the DCP | $ 32.3 | $ 32.9 | $ 26.8
Employee Stock Option |  |  |
Stock Incentive Plans |  |  |
Securities available for future issuance (in shares) | 12.6 |  |
Expiration term of unexercised options | 7 years |  |
Number of Options |  |  |
Balance at the beginning of the period (in shares) | 0.7 | 0.9 | 1.3
Exercised (in shares) | (0.1) | (0.2) | (0.4)
Balance at the end of the period (in shares) | 0.6 | 0.7 | 0.9
Exercisable at the end of the period (in shares) |  | 0.1 | 0.3
Stock options, Weighted Average Exercise Price |  |  |
Balance at the beginning of the period (in dollars per share) | $ 31.11 | $ 30.36 | $ 28.26
Exercised (in dollars per share) | 27.79 | 26.42 | 23.96
Balance at the end of the period (in dollars per share) | $ 31.62 | 31.11 | 30.36
Exercisable at the end of the period (in dollars per share) |  | $ 27.79 | $ 26.99

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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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