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Fair Value Measurements - Summary of Investment Portfolio (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2022 | Mar. 31, 2021 | Mar. 31, 2022 | Mar. 31, 2021 | Sep. 30, 2021
Fair Value Disclosures [Abstract] |  |  |  |  |  |  |  |
Original cost |  | $ 335 |  | $ 335 |  |  | $ 335 |
Accumulated adjustments, net |  | 37 |  | 37 |  |  | (50) |
Adjusted cost | [1] | 372 |  | 372 |  |  | 285 |
Investment income |  | 2 | $ 3 | 10 | [2] | $ 8 | 12 | [2]
Net unrealized gain | [3] |  |  | 24 |  |  | 49 |
Realized gains | [3] |  |  | 1 |  |  | 17 |
Funds restricted for specific transactions | [4] |  |  | 44 |  |  | 91 |
Disbursements |  | (21) | $ (10) | (28) |  | $ (18) | (33) |
Fair value |  | $ 423 |  | $ 423 |  |  | $ 421 |
[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
[2] | Investment income relates to the demand deposit and includes interest income as well as dividend income transferred from the equity and fixed income mutual funds
[3] | Presented under the original cost method
[4] | The March 31, 2022 period included additional contributions to the Environmental trust from proceeds associated with the Performance Adhesives sale and excess land sales. The September 30, 2021 period included $90 million to establish the Environmental trust.

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