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Income Taxes (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2018 | Mar. 31, 2018 | Mar. 31, 2019 | Mar. 31, 2018 | Sep. 30, 2018
Income Taxes |  |  |  |  |
Effective tax rate (as a percent) |  |  | (8.70%) | 130.40% |
Net benefit from Tax Cuts and Jobs Acts of 2017 |  |  |  | $ 41.7 |
Benefits from change in uncertain tax positions in US and Canada |  |  |  | 36.0 |
Income tax credits and incentives |  |  | $ 12.3 | 13.7 |
State income tax expense |  |  | $ 7.4 |  |
Goodwill impairment charge |  |  |  | $ 33.9 |
Federal statutory rate (as a percent) |  |  | 21.00% | 24.50% | 35.00%
Valuation allowance on foreign tax credits | $ 38.1 |  | $ 38.1 |  |
Amount of uncertain tax positions for future years |  | $ 26.2 |  |  |
Gross book-tax differences from non-U.S. subsidiaries |  |  | $ 1,800.0 |  |

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