Investments - Narrative (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | |||
|---|---|---|---|---|---|
Mar. 31, 2026 |
Mar. 31, 2025 |
Mar. 31, 2026 |
Mar. 31, 2025 |
Sep. 30, 2025 |
|
| Investments, Debt and Equity Securities [Abstract] | |||||
| Cumulative upward adjustments from observed price | $ 23 | $ 23 | $ 23 | ||
| Cumulative downward adjustments from impairments | 10 | 10 | $ 10 | ||
| Unrealized losses on investments | $ 0 | $ 3 | $ 0 | $ 3 | |