Fair Value Measurements and Investments - Schedule of Non-Marketable Equity Securities (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | |||
|---|---|---|---|---|---|
Mar. 31, 2025 |
Mar. 31, 2024 |
Mar. 31, 2025 |
Mar. 31, 2024 |
Sep. 30, 2024 |
|
| Fair Value Disclosures [Abstract] | |||||
| Initial cost basis | $ 702 | $ 702 | $ 711 | ||
| Adjustments: | |||||
| Upward adjustments | 850 | 850 | 910 | ||
| Downward adjustments, including impairment | (494) | (494) | (465) | ||
| Carrying amount | 1,058 | 1,058 | $ 1,156 | ||
| Upward adjustments | 7 | $ 0 | 7 | $ 9 | |
| Downward adjustments, including impairment | $ (31) | $ (15) | $ (49) | $ (15) | |