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Supplemental Financial Statement Information - Schedule of Other Income, Net (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2020 | Jun. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Other Income and Expenses [Abstract] |  |  |  |  |
Rent and royalty income | $ 0.1 |  | $ 1.4 | $ 0.6 | $ 2.8
Gains from disposal of property, plant and equipment, net | 0.8 |  | 62.3 | 3.3 | 90.6
Net equity (loss) gain on joint ventures | (1.0) |  | (2.4) | (6.6) | (8.7)
Loss from sale of businesses | 0.0 |  | 5.8 | 0.0 | (1.9)
Adjustment to indemnification for conditional ARO costs | 0.0 |  | 0.0 | 4.3 | 0.0
Other | (0.3) |  | 0.1 | 0.0 | 0.1
Other income (expense), net | (0.4) |  | 67.2 | (0.8) | 82.9
Allegheny & Tsingshan Stainless | Employee Severance [Member] |  |  |  |  |
Other Income and Expenses [Abstract] |  |  |  |  |
Net equity (loss) gain on joint ventures | $ 0.0 | $ (2.4) | $ 0.0 | $ (2.4) | $ 0.0

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