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Income Taxes - Additional information (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Income taxes |  |  |  |
Valuation allowance related to net operating losses and other deferred tax assets | $ 217.5 | $ 120.6 |  |
Federal corporate tax rate (as a percent) | 21.00% | 21.00% | 24.50% | 35.00%
Provisional income tax (benefit) |  |  | $ 38.9 |
Undistributed earnings from foreign subsidiaries |  |  | 79.8 |
One-time transition tax on accumulated foreign earnings |  |  | 53.4 |
Change in uncertain tax positions |  | $ (1.5) | 12.5 |
Tax benefit from income tax examination |  |  | 27.7 |
Tax benefit from income tax examination, R&D credits |  |  | 19.9 |
Tax benefit from foreign subsidiaries | $ (55.7) | (41.7) | (51.1) |
Change in valuation allowance | 96.9 |  |  |
Amount of uncertain tax positions for future years |  |  | 19.9 |
United States |  |  |  |
Income taxes |  |  |  |
Valuation allowance related to net operating losses and other deferred tax assets |  |  | $ 38.1 |
Tax benefit from income tax examination |  | $ (38.1) |  |
Canada |  |  |  |
Income taxes |  |  |  |
Valuation allowance related to net operating losses and other deferred tax assets | 31.7 |  |  |
Change in valuation allowance | $ 31.7 |  |  |

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