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Fair Value Measurements - Summary of Investment Portfolio (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Sep. 30, 2018
Fair Value Disclosures [Abstract] |  |  |  |  |  |  |  |
Original cost |  | $ 335 |  | $ 335 |  |  | $ 335 |
Accumulated adjustments, net |  | (47) |  | (47) |  |  | (38) |
Adjusted cost | [1] | 288 |  | 288 |  |  | 297 |
Investment income |  | 2 | $ 2 | 7 | [2] | $ 6 | 8 | [2]
Net unrealized gain | [3] |  |  | 31 |  |  | 54 |
Realized gains | [3] |  |  | 26 |  |  | 6 |
Settlement funds |  |  |  | 2 |  |  | 10 |
Disbursements |  | (5) | $ (7) | (25) |  | $ (25) | (33) |
Fair value |  | $ 329 |  | $ 329 |  |  | $ 342 |
[1] | The accumulated adjustments include investment income, realized gains, disbursements and settlements recorded in previous periods.
[2] | Investment income for the demand deposit 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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