Fair Value Measurements - Summary of Investment Income, Net Gains and Losses, Funds restricted for Specific Transactions and Disbursements Related to Investments (Details) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | ||||
|---|---|---|---|---|---|---|
Jun. 30, 2025 |
Jun. 30, 2024 |
Jun. 30, 2025 |
Jun. 30, 2024 |
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| Fair Value Disclosures [Abstract] | ||||||
| Investment income | [1] | $ 3 | $ 3 | $ 10 | $ 12 | |
| Net gains | [1] | 19 | 1 | 5 | 39 | |
| Funds restricted for specific transactions | 0 | 0 | 8 | 5 | ||
| Disbursements | $ (17) | $ (23) | $ (41) | $ (60) | ||
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