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Accounting Policies (Accounting Pronouncements) (Details) $ in Millions | 3 Months Ended | 9 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2020 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2020 USD ($) segment | Sep. 30, 2019 USD ($) | Jan. 01, 2021 USD ($) | Jun. 30, 2020 USD ($) | Dec. 31, 2019 USD ($) employee | Jun. 30, 2019 USD ($) | Dec. 31, 2018 USD ($)
Revenue recognition [Line Items] |  |  |  |  |  |  |  |  |
Number of business segments | segment |  |  | 2 |  |  |  |  |  |
Impact of new accounting principle on prior period operating income | $ (9.6) | $ 94.5 | $ (214.6) | $ 267.6 |  |  |  |  |
Unfavorable impact on pre-tax operating results | 46.2 | (119.0) | 339.1 | (220.4) |  |  |  |  |
Entity Number Of Employees Covered By USW Collective Bargaining Agreements Expiring February 29 2020 | employee |  |  |  |  |  |  | 1,500 |  |
Equity component of convertible note | 1.1 |  | 49.8 |  |  |  |  |  |
Equity adjustment for adoption | 1,849.8 | 2,279.9 | 1,849.8 | 2,279.9 |  | $ 1,842.3 | $ 2,193.2 | $ 2,138.6 | $ 1,991.6
[AdditionalPaidInCapitalMember] |  |  |  |  |  |  |  |  |
Revenue recognition [Line Items] |  |  |  |  |  |  |  |  |
Equity component of convertible note | 1.1 |  | 49.8 |  |  |  |  |  |
Equity adjustment for adoption | 1,622.2 | 1,612.1 | 1,622.2 | 1,612.1 |  | 1,617.8 | 1,618.0 | 1,604.6 | 1,615.4
[RetainedEarningsMember] |  |  |  |  |  |  |  |  |
Revenue recognition [Line Items] |  |  |  |  |  |  |  |  |
Equity adjustment for adoption | $ 1,227.4 | $ 1,622.8 | $ 1,227.4 | $ 1,622.8 |  | $ 1,277.6 | $ 1,679.3 | $ 1,511.8 | $ 1,422.0
Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt |  |  |  |  |  |  |  |  |
Revenue recognition [Line Items] |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.50% |  | 3.50% |  |  |  |  |  |
Forecast | Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt | [RetainedEarningsMember] | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |  |  |
Revenue recognition [Line Items] |  |  |  |  |  |  |  |  |
Equity adjustment for adoption |  |  |  |  | $ (4.6) |  |  |  |
Allegheny & Tsingshan Stainless |  |  |  |  |  |  |  |  |
Revenue recognition [Line Items] |  |  |  |  |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  | 50.00% |  |  |  |  |  |
Uniti |  |  |  |  |  |  |  |  |
Revenue recognition [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] |  |  |  |  |  |  |  |  |
Revenue recognition [Line Items] |  |  |  |  |  |  |  |  |
Joint venture ownership percentage | 60.00% |  | 60.00% |  |  |  |  |  |

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