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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, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019 | Sep. 30, 2019
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |  |  |
Original cost |  | $ 335 |  | $ 335 |  |  | $ 335 |
Accumulated adjustments, net | [1] | (30) |  | (30) |  |  | (47) |
Adjusted cost |  | 305 |  | 305 |  |  | 288 |
Investment income |  | 2 | $ 2 | 8 | [2] | $ 7 | 10 | [2]
Net unrealized gain | [3] |  |  | 37 |  |  | 29 |
Realized gains | [3] |  |  | 0 |  |  | 32 |
Settlement funds |  |  |  | 3 |  |  | 7 |
Disbursements |  | (7) | $ (5) | (26) |  | $ (25) | (32) |
Fair value |  | 327 |  | 327 |  |  | 334 |
Available-for-sale Securities [Member] |  |  |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |  |  |
Adjusted cost |  | 292 |  | 292 |  |  | 305 |
Fair value |  | 327 |  | 327 |  |  | 334 |
Gross Unrealized Gain |  | 39 |  | 39 |  |  | 31 |
Gross Unrealized Loss |  | (4) |  | (4) |  |  | (2) |
Fixed Income Mutual Fund [Member] |  |  |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |  |  |
Adjusted cost |  | 166 |  | 166 |  |  | 176 |
Fair value |  | 181 |  | 181 |  |  | 183 |
Gross Unrealized Gain |  | 15 |  | 15 |  |  | 7 |
Gross Unrealized Loss |  | 0 |  | 0 |  |  | 0 |
Equity Mutual Fund [Member] |  |  |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |  |  |
Adjusted cost |  | 116 |  | 116 |  |  | 120 |
Fair value |  | 136 |  | 136 |  |  | 142 |
Gross Unrealized Gain |  | 24 |  | 24 |  |  | 24 |
Gross Unrealized Loss |  | (4) |  | (4) |  |  | (2) |
Demand Deposit [Member] |  |  |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |  |  |
Adjusted cost |  | 10 |  | 10 |  |  | 9 |
Fair value |  | $ 10 |  | $ 10 |  |  | $ 9 |
[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 fixed income mutual funds.
[3] | Presented under the original cost method.

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