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Stock Incentive Plans - Components of Pretax Stock-Based Award (Net of Forfeitures) and Associated Income Tax Benefits (Parenthetical) (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Stock-based compensation | $ 26 | [1] | $ 44 | [2] | $ 28 | [3]
Cash-settled Nonvested Stock Awards [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Stock-based compensation | 5 |  | 9 |  | 5 |
Cash-settled Performance Shares [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Stock-based compensation | $ 0 |  | $ 8 |  | $ 3 |
[1] | The year ended September 30, 2019 included $5 million and zero of expense related to cash-settled nonvested restricted stock awards and cash-settled performance units, respectively.
[2] | The year ended September 30, 2018 included $9 million and $8 million of expense related to cash-settled nonvested restricted stock awards and cash-settled performance units, respectively.
[3] | The year ended September 30, 2017 included $5 million and $3 million of expense related to cash-settled nonvested restricted stock awards and cash-settled performance units, respectively.

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