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Joint Ventures (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 10 Months Ended | 42 Months Ended |
--- | --- | --- | --- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Dec. 31, 2018 | Sep. 30, 2021 | Dec. 31, 2020
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Cash and cash equivalents | $ 472.5 |  | $ 472.5 |  |  |  | $ 645.9
Income (loss) from equity investments | 0.0 | $ (1.9) | 0.0 | $ (5.6) |  |  |
Next Gen Alloys LLC |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Cash and cash equivalents | $ 1.7 |  | $ 1.7 |  |  |  |
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 |  |
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) |  |  |
Allegheny & Tsingshan Stainless | Forecast |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Sales to noncontrolling interests |  |  |  |  |  | $ 17.5 |
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 | 47.2 |  | 47.2 |  |  |  |
Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Income (loss) from equity investments | (0.4) | (4.6) | (0.9) | (8.5) |  |  |
Due from Affiliates | 13.2 |  | 13.2 |  |  |  | 14.0
Advanced Alloys & Solutions [Member] | Uniti |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Income (loss) from equity investments | 0.4 | $ 0.3 | 0.9 | $ 0.5 |  |  |
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 |  | 0.5 |  |  |  | 0.5
Other Noncurrent Assets [Member] | Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Due from Affiliates | $ 12.7 |  | $ 12.7 |  |  |  | $ 13.5
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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