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Income Taxes - Additional information (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Income taxes |  |  |  |
Valuation allowance |  | $ 169.1 | $ 197.1 |
Federal corporate tax rate (as a percent) |  | 21.00% | 24.50% | 35.00%
Provisional income tax (benefit) |  |  | $ (32.0) |
Undistributed earnings from foreign subsidiaries |  |  | 79.8 |
One-time transition tax on accumulated foreign earnings |  |  | 64.0 |
Tax benefit from foreign earnings and profits |  | $ (1.5) |  |
Tax benefit from income tax examination |  |  | 27.7 |
Tax benefit from income tax examination, R&D credits | $ 26.2 |  |  |
Tax benefit from foreign subsidiaries |  | (43.2) | (47.1) | $ (29.3)
Change in valuation allowance |  | 28.0 |  |
Amount of uncertain tax positions for future years | $ 26.2 |  |  |
United States |  |  |  |
Income taxes |  |  |  |
Valuation allowance |  |  | 38.1 |
Tax benefit from income tax examination |  | (38.1) |  |
Canada |  |  |  |
Income taxes |  |  |  |
Valuation allowance |  | $ 6.0 |  |
Change in valuation allowance |  |  | $ 13.1 |

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