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Income Taxes - Gross Unrecognized (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2024 | Sep. 30, 2023
Reconciliation of the beginning and ending amount of gross unrecognized tax benefits |  |
Balance at the beginning of the year | $ 62.1 | $ 55.2
Gross increase in current period's tax positions | 34.5 | 3.5
Gross increase in prior years' tax positions | 13.9 | 17.9
Gross decrease in prior years' tax positions | (20.1) | (13.3)
Decrease due to settlement with tax authorities | (7.4) | (1.0)
Decrease due to lapse of statute of limitations | (2.2) |
Gross change due to foreign exchange fluctuations | 0.5 | (0.2)
Balance at the end of the year | 81.3 | 62.1
Income Taxes, additional disclosures |  |
Accrued interest, excluding any related income tax benefits | 27.7 | 24.4
Accrued penalties, excluding any related income tax benefits | $ 3.5 | $ 1.5

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