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Fair Value Measurements - Summary of Available-for-sale Securities Portfolio (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Original cost |  | $ 335 |  | $ 335 |  |
Accumulated adjustments, net |  | (47) |  | (38) |  |
Adjusted cost | [1] | 288 |  | 297 |  |
Investment income |  | 10 | [2] | 8 | [2] | $ 9
Net unrealized gain | [3] | 29 |  | 54 |  |
Realized gain | [3] | 32 |  | 6 |  |
Settlement funds |  | 7 |  | 10 |  |
Disbursements |  | (32) |  | (33) |  | $ (27)
Fair value |  | 334 |  | 342 |  |
Available-for-sale Securities [Member] |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Adjusted cost |  | 305 |  | 288 |  |
Fair value |  | 334 |  | 342 |  |
Unrealized gain |  | 31 |  | 59 |  |
Unrealized loss |  | (2) |  | (5) |  |
Bond Mutual Fund [Member] |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Adjusted cost |  | 176 |  | 120 |  |
Fair value |  | 183 |  | 115 |  |
Unrealized gain |  | 7 |  | 0 |  |
Unrealized loss |  | 0 |  | (5) |  |
Equity Mutual Fund [Member] |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Adjusted cost |  | 120 |  | 148 |  |
Fair value |  | 142 |  | 207 |  |
Unrealized gain |  | 24 |  | 59 |  |
Unrealized loss |  | (2) |  | 0 |  |
Demand Deposits [Member] |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Demand Deposit |  | $ 9 |  | $ 20 |  |
[1] | The adjusted cost of the demand deposit includes accumulated investment income, realized gains, disbursements and settlements recorded in previous periods.
[2] | Investment income relates to the demand deposit and includes interest income as well as dividend income transferred from the equity and corporate bond mutual funds.
[3] | Presented under the original cost method.

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