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Joint Ventures (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 10 Months Ended | 34 Months Ended
--- | --- | --- | --- | ---
Sep. 30, 2020 | Jun. 30, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2018 | Dec. 31, 2020
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Cash and cash equivalents | $ 572.2 |  | $ 490.8 |  | $ 572.2 |  |  |
Income (loss) from equity investments | (1.0) |  |  | $ (2.4) | (6.6) | $ (8.7) |  |
Next Gen Alloys LLC |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Cash and cash equivalents | $ 3.2 |  |  |  | $ 3.2 |  |  |
Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  |  |  | 50.00% |  |  |
Sales to noncontrolling interests |  |  |  |  |  |  | $ 12.0 |
Charges for termination benefits |  | $ 4.8 |  |  |  |  |  |
Impairment of Long-Lived Assets Held-for-use |  |  | 14.2 |  |  |  |  |
Due From Affiliates, Allowance For Credit Loss |  |  | 4.3 |  |  |  |  |
Allegheny & Tsingshan Stainless | Employee Severance [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Income (loss) from equity investments | $ 0.0 | $ (2.4) |  | 0.0 | $ (2.4) | 0.0 |  |
Uniti |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  |  |  | 50.00% |  |  |
Tsingshan Group | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Due from Affiliates | $ 5.5 |  |  |  | $ 5.5 |  |  |
Advanced Alloys & Solutions [Member] | Shanghai STAL Precision Stainless Steel Co Ltd |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Cash and cash equivalents | 48.9 |  |  |  | 48.9 |  |  |
Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Income (loss) from equity investments | (1.6) |  |  | (2.6) | (10.1) | (9.9) |  |
Due from Affiliates | 14.5 |  | 32.5 |  | 14.5 |  |  |
Advanced Alloys & Solutions [Member] | Uniti |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Income (loss) from equity investments | 0.6 |  |  | $ 0.2 | 1.1 | $ 1.2 |  |
Forecast | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Sales to noncontrolling interests |  |  |  |  |  |  |  | $ 17.5
Property, Plant and Equipment [Member] | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Income (loss) from equity investments |  |  | (7.1) |  |  |  |  |
Prepaid expenses and other current assets | Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Due from Affiliates | 0.5 |  | 8.3 |  | 0.5 |  |  |
Other Noncurrent Assets [Member] | Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Due from Affiliates | $ 14.0 |  | $ 24.2 |  | $ 14.0 |  |  |
Allegheny Technologies Inc | Next Gen Alloys LLC |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Joint venture ownership percentage | 51.00% |  |  |  | 51.00% |  |  |
Allegheny Technologies Inc | Advanced Alloys & Solutions [Member] | Shanghai STAL Precision Stainless Steel Co Ltd |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Joint venture ownership percentage | 60.00% |  |  |  | 60.00% |  |  |
China Baowu Steel Group Corporation Limited | Advanced Alloys & Solutions [Member] | China Baowu Steel Group Corporation Limited | Shanghai STAL Precision Stainless Steel Co Ltd |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Joint venture ownership percentage by unaffiliated entity | 40.00% |  |  |  | 40.00% |  |  |
Tsingshan Group | Tsingshan Group | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Joint venture ownership percentage by unaffiliated entity | 50.00% |  |  |  | 50.00% |  |  |
VSMPO | VSMPO | Uniti |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Joint venture ownership percentage by unaffiliated entity | 50.00% |  |  |  | 50.00% |  |  |

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