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Business Segments - Additional Information (Details) | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2021 USD ($) | Jun. 30, 2020 USD ($) | Jun. 30, 2021 USD ($) segment | Jun. 30, 2020 USD ($) | Dec. 31, 2020 USD ($)
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Number of business segments | segment |  |  | 2 |  |
Strike related costs | $ 40,300,000 | $ 0 | $ 40,300,000 | $ 0 |
Impairment of goodwill | 0 | (287,000,000.0) | 0 | (287,000,000.0) |
Debt extinguishment charge (See Note 7) | 0 | $ (21,500,000) | 0 | (21,500,000) |
Gain on the sale of oil and gas rights |  |  |  | $ 2,500,000 |
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Debt, Gross | 291,400,000 |  | $ 291,400,000 |  | $ 291,400,000
High Performance Materials & Components | Operating Segments |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Strike related costs | 2,100,000 |  |  |  |
Advanced Alloys & Solutions [Member] | Operating Segments |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Strike related costs | $ 38,200,000 |  |  |  |

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