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v3.20.2

Business Segments - Additional Information (Details) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2020 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2020 USD ($) segment | Sep. 30, 2019 USD ($) | Dec. 31, 2019 USD ($)
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Restructuring reserve | $ 20.1 |  | $ 20.1 |  | $ 4.5
Gain on asset sales, net | 0.0 | $ 68.2 | $ 2.5 | $ 89.8 |
Number of business segments | segment |  |  | 2 |  |
Goodwill | 238.4 |  | $ 238.4 |  | $ 525.8
Adjustment to indemnification for conditional ARO costs | 0.0 | 0.0 | 4.3 | 0.0 |
Gain on the sale of oil and gas rights |  | 62.4 | 2.5 | 91.7 |
Loss from sale of businesses | $ 0.0 | 5.8 | 0.0 | (1.9) |
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal |  |  | $ 0.0 | (1.8) |
Cast Products [Member] |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal |  | 6.2 |  | 6.2 |
Forged Products |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal |  | $ (0.4) |  | $ (8.1) |
Aerospace and Defense Market Concentration [Member] | High Performance Materials & Components | Revenue from Contract with Customer, Segment Benchmark [Member] |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Concentration Risk, Percentage |  |  | 80.00% |  |
Energy and Aerospace & Defense Market Concentration [Member] | Advanced Alloys & Solutions [Member] | Revenue from Contract with Customer, Segment Benchmark [Member] |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Concentration Risk, Percentage |  |  | 50.00% |  |
Allegheny & Tsingshan Stainless |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  | 50.00% |  |
Uniti |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  | 50.00% |  |
Allegheny Technologies Inc | Shanghai STAL Precision Stainless Steel Co Ltd | Advanced Alloys & Solutions [Member] |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Joint venture ownership percentage | 60.00% |  | 60.00% |  |

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