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Income Taxes - Net Unrecognized Tax Benefits (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
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Income Tax Disclosure [Abstract] |  |  |  |
Deferred income taxes | $ 7.8 | $ 15.5 |  |
Other noncurrent liabilities | 1.3 | 1.0 |  |
Net unrecognized tax benefits | 9.1 | 16.5 |  |
Gross unrecognized tax benefits | $ 9.1 | $ 16.5 | $ 14.0 | $ 12.7

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