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Fair Value Measurements - Summary of Available-for-sale Securities Portfolio (Details) - USD ($) $ in Millions | Mar. 31, 2022 | Sep. 30, 2021
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |  |
Adjusted cost | [1] | $ 372 | $ 285
Fair Value |  | 423 | 421
Restricted Securities [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Adjusted cost |  | 399 | 372
Gross Unrealized Gain |  | 41 | 51
Gross Unrealized Loss |  | (17) | (2)
Fair Value |  | 423 | 421
Fixed Income Mutual Fund [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Adjusted cost |  | 235 | 223
Gross Unrealized Gain |  | 0 | 7
Gross Unrealized Loss |  | (16) | (1)
Fair Value |  | 219 | 229
Equity Mutual Fund [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Adjusted cost |  | 160 | 143
Gross Unrealized Gain |  | 41 | 44
Gross Unrealized Loss |  | (1) | (1)
Fair Value |  | 200 | 186
Demand Deposit [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Adjusted cost |  | 4 | 6
Gross Unrealized Gain |  | 0 | 0
Gross Unrealized Loss |  | 0 | 0
Fair Value |  | $ 4 | $ 6
[1] | The adjusted cost of the demand deposits includes accumulated investment income, realized gains, additional funds restricted for specific transactions and disbursements recorded in previous periods. The adjusted cost as of March 31, 2022 includes the $90 million funding to establish the Environmental trust

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