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Stockholders' Deficit - Narrative (Details) $ / shares in Units, $ in Millions |  | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jul. 30, 2025 USD ($) | Sep. 30, 2025 USD ($) vote $ / shares | Jun. 30, 2025 $ / shares | Sep. 30, 2024 USD ($) $ / shares | Sep. 30, 2025 USD ($) vote $ / shares | Sep. 30, 2024 USD ($) $ / shares | Mar. 31, 2025 USD ($)
Dividends Payable [Line Items] |  |  |  |  |  |  |
Number of votes per share of common stock permitted on proposals presented to stockholders (vote) | vote |  | 1 |  |  | 1 |  |
Cash dividends declared, per common share (in dollars per share) | $ / shares |  | $ 0.82 | $ 0.71 | $ 0.71 | $ 1.53 | $ 1.33 |
Excise taxes |  |  |  |  | $ 10 |  | $ 26
Excise tax payment | $ 26 |  |  |  |  |  |
Share Repurchase Transactions |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |
Excise taxes |  | $ 8 |  | $ 15 | $ 10 | $ 16 |

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