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Joint Ventures (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 10 Months Ended | 58 Months Ended
--- | --- | --- | --- | ---
Sep. 30, 2023 | Dec. 31, 2022 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022 | Dec. 31, 2018 | Dec. 31, 2022
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Cash and cash equivalents | $ 432.9 | $ 584.0 |  | $ 432.9 |  |  | $ 584.0
Sale of noncontrolling interests |  |  |  | 0.0 | $ 0.9 |  |
Income (loss) from equity investments | (0.7) |  | $ 0.8 | (1.0) | 11.2 |  |
Allegheny & Tsingshan Stainless |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Tariff refunds receivable |  |  | 19.7 |  | 19.7 |  |
Allegheny & Tsingshan Stainless | A&T Stainless |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Income (loss) from equity investments |  |  |  |  | 9.9 |  |
Next Gen Alloys LLC |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Cash and cash equivalents | $ 1.0 |  |  | $ 1.0 |  |  |
Sale of noncontrolling interests |  |  |  |  | 0.9 |  |
Allegheny & Tsingshan Stainless |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  |  | 50.00% |  |  |
Allegheny & Tsingshan Stainless | Tsingshan Group |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Sale of noncontrolling interests |  | 5.5 |  |  |  | $ 12.0 | 17.5
Uniti |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  |  | 50.00% |  |  |
Advanced Alloys & Solutions | Shanghai STAL Precision Stainless Steel Co Ltd |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Cash and cash equivalents | $ 85.0 |  |  | $ 85.0 |  |  |
Advanced Alloys & Solutions | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Income (loss) from equity investments | (0.5) |  | (0.5) | (1.3) | 8.7 |  |
Advanced Alloys & Solutions | Uniti |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Income (loss) from equity investments | (0.2) |  | $ 1.3 | 0.3 | $ 2.5 |  |
Advanced Alloys & Solutions | Related Party | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Other receivables | 1.9 | 3.2 |  | 1.9 |  |  | 3.2
Prepaid expenses and other current assets | Advanced Alloys & Solutions | Related Party | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Other receivables | 0.5 | 0.4 |  | 0.5 |  |  | 0.4
Other Noncurrent Assets [Member] | Advanced Alloys & Solutions | Related Party | Allegheny & Tsingshan Stainless |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Other receivables | $ 1.4 | $ 2.8 |  | $ 1.4 |  |  | $ 2.8
China Baowu Steel Group Corporation Limited | Advanced Alloys & Solutions | 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% |  |  |
ATI Inc. | Next Gen Alloys LLC |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Joint venture ownership percentage | 51.00% |  |  | 51.00% |  |  |
ATI Inc. | Advanced Alloys & Solutions | Shanghai STAL Precision Stainless Steel Co Ltd |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Joint venture ownership percentage | 60.00% |  |  | 60.00% |  |  |

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