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Accounting Policies (Accounting Pronouncements) (Details) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2024 USD ($) segment | Apr. 02, 2023 USD ($) | Dec. 31, 2023 USD ($) | Jan. 01, 2023 USD ($)
Revenue recognition [Line Items] |  |  |  |
Number of business segments | segment | 2 |  |  |
Impact of new accounting principle on prior period operating income | $ 115.2 | $ 112.6 |  |
Unfavorable impact on pre-tax operating results | (85.3) | (90.9) |  |
Equity adjustment for adoption | 1,373.2 | 1,214.3 | $ 1,480.5 | $ 1,157.2
Additional Paid-in Capital [Member] |  |  |  |
Revenue recognition [Line Items] |  |  |  |
Equity adjustment for adoption | 1,703.1 | 1,675.1 | 1,697.1 | 1,668.1
[RetainedEarningsMember] |  |  |  |
Revenue recognition [Line Items] |  |  |  |
Equity adjustment for adoption | $ (4.0) | $ (396.4) | $ (70.1) | $ (480.9)
ATI Inc. 3.5% Convertible Senior Notes due 2025 | Convertible Debt |  |  |  |
Revenue recognition [Line Items] |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage | 3.50% |  | 3.50% |
Allegheny & Tsingshan Stainless |  |  |  |
Revenue recognition [Line Items] |  |  |  |
Equity method investment ownership percentage | 50.00% |  |  |
Uniti |  |  |  |
Revenue recognition [Line Items] |  |  |  |
Equity method investment ownership percentage | 50.00% |  |  |

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