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Accounting Policies (Accounting Pronouncements) (Details) $ in Millions | 3 Months Ended | 6 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jun. 30, 2021 USD ($) | Jun. 30, 2020 USD ($) | Jun. 30, 2021 USD ($) segment | Jun. 30, 2020 USD ($) | Mar. 31, 2021 USD ($) | Jan. 01, 2021 USD ($) | Dec. 31, 2020 USD ($) | Mar. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
Revenue recognition [Line Items] |  |  |  |  |  |  |  |  |
Number of business segments | segment |  |  | 2 |  |  |  |  |  |
Impact of new accounting principle on prior period operating income | $ (11.3) | $ (273.4) | $ 20.5 | $ (205.0) |  |  |  |  |
Unfavorable impact on pre-tax operating results | 40.4 | 327.3 | 37.3 | 292.9 |  |  |  |  |
Equity adjustment for adoption | 606.1 | 1,842.3 | 606.1 | 1,842.3 | $ 609.3 |  | $ 641.4 | $ 2,192.6 | $ 2,193.2
Additional Paid-in Capital [Member] |  |  |  |  |  |  |  |  |
Revenue recognition [Line Items] |  |  |  |  |  |  |  |  |
Equity adjustment for adoption | 1,587.5 | 1,617.8 | 1,587.5 | 1,617.8 | 1,580.5 |  | 1,625.5 | 1,592.1 | 1,618.0
[RetainedEarningsMember] |  |  |  |  |  |  |  |  |
Revenue recognition [Line Items] |  |  |  |  |  |  |  |  |
Equity adjustment for adoption | $ 53.8 | $ 1,277.6 | $ 53.8 | $ 1,277.6 | $ 103.0 |  | $ 106.5 | $ 1,700.4 | $ 1,679.3
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% |  |  |  | 3.50% |  |
Accounting Standards Update 2020-06 | Additional Paid-in Capital [Member] |  |  |  |  |  |  |  |  |
Revenue recognition [Line Items] |  |  |  |  |  |  |  |  |
Equity adjustment for adoption |  |  |  |  |  | $ (49.8) |  |  |
Accounting Standards Update 2020-06 | [RetainedEarningsMember] |  |  |  |  |  |  |  |  |
Revenue recognition [Line Items] |  |  |  |  |  |  |  |  |
Equity adjustment for adoption |  |  |  |  |  | 4.4 |  |  |
Accounting Standards Update 2020-06 | Allegheny Technologies 3.5% Convertible Senior Notes due 2025 | Convertible Debt |  |  |  |  |  |  |  |  |
Revenue recognition [Line Items] |  |  |  |  |  |  |  |  |
Equity adjustment for adoption |  |  |  |  |  | $ (45.4) |  |  |
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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