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Joint Ventures (Details) - USD ($) $ in Millions | 3 Months Ended | 10 Months Ended | 12 Months Ended | 58 Months Ended
--- | --- | --- | --- | ---
Dec. 31, 2022 | Dec. 31, 2018 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2022
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Cash and cash equivalents | $ 584.0 |  | $ 584.0 | $ 687.7 |  | $ 584.0
Income (loss) from equity method investments |  |  | 12.6 | 0.1 | $ (7.0) |
Undistributed earnings of investees accounted for under equity method | 7.8 |  | 7.8 |  |  | 7.8
Sale to noncontrolling interests |  |  | 6.4 | 0.0 | 0.0 |
Sales of subsidiary shares to noncontrolling interest |  |  | 0.9 |  |  |
Noncontrolling Interests |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Sales of subsidiary shares to noncontrolling interest |  |  | 0.9 |  |  |
Shanghai STAL Precision Stainless Steel Co Ltd [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Cash and cash equivalents | 53.5 |  | 53.5 |  |  | 53.5
Next Gen Alloys LLC [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Cash and cash equivalents | $ 1.2 |  | $ 1.2 |  |  | $ 1.2
Allegheny & Tsingshan Stainless |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  | 50.00% |  |  | 50.00%
Income (loss) from equity method investments |  |  | $ 9.1 | (0.9) | (10.6) |
Revenue from related parties |  |  | 0.0 | 0.0 | 18.4 |
Business Exit Costs |  |  | $ (1.8) |  | 4.8 |
Uniti [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  | 50.00% |  |  | 50.00%
Revenue from related parties |  |  | $ 45.0 | 45.8 | 36.7 |
Related party accounts receivable | $ 4.5 |  | 4.5 | 6.1 |  | $ 4.5
Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Income (loss) from equity method investments |  |  | 8.2 | (0.9) | (8.2) |
Due from affiliates | 3.2 |  | 3.2 | 2.9 |  | 3.2
Advanced Alloys & Solutions [Member] | Uniti [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Income (loss) from equity method investments |  |  | 4.4 | 1.0 | $ 1.2 |
Prepaid expenses and other current assets [Member] | Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Due from affiliates | 0.4 |  | 0.4 | 0.7 |  | 0.4
Other Noncurrent Assets [Member] | Advanced Alloys & Solutions [Member] | Allegheny & Tsingshan Stainless |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Due from affiliates | $ 2.8 |  | $ 2.8 | $ 2.2 |  | $ 2.8
Allegheny Technologies Inc | Shanghai STAL Precision Stainless Steel Co Ltd [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Joint venture ownership percentage | 60.00% |  | 60.00% |  |  | 60.00%
Allegheny Technologies Inc | Next Gen Alloys LLC [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Joint venture ownership percentage | 51.00% |  | 51.00% |  |  | 51.00%
Allegheny Technologies Inc | China Baowu Steel Group Corporation Limited [Member] | Shanghai STAL Precision Stainless Steel Co Ltd [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Joint venture ownership percentage by unaffiliated entity | 40.00% |  | 40.00% |  |  | 40.00%
VSMPO | VSMPO | Uniti [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Joint venture ownership percentage by unaffiliated entity | 50.00% |  | 50.00% |  |  | 50.00%
Tsingshan Group | Allegheny & Tsingshan Stainless |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  | 50.00% |  |  | 50.00%
Sale to noncontrolling interests | $ 5.5 | $ 12.0 |  |  |  | $ 17.5
Allegheny & Tsingshan Stainless |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Income from Tariff Refunds |  |  | $ 19.7 |  |  |
Allegheny & Tsingshan Stainless | Import Duties |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Income (loss) from equity method investments |  |  | $ 9.9 |  |  |

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