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Fair Value of Financial Instruments (Details) - USD ($) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | Jul. 02, 2020
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items] |  |  |  |
Derivative Asset Statement Of Financial Position Extensible Enumeration Not Disclosed Flag | Assets |  |  |
Derivative Liability Statement Of Financial Position Extensible Enumeration Not Disclosed Flag | Liabilities |  |  |
ATI Convertible Senior Notes, 3.5%, Due 2025 [Member] | Convertible Debt [Member] |  |  |  |
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items] |  |  |  |
Equity component of convertible debt |  |  | $ 46.8 | $ 51.4
Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | $ 584.0 | $ 687.7 |  |
Derivative financial instruments: Assets | 0.0 | 0.0 |  |
Derivative financial instruments: Liabilities | 0.0 | 0.0 |  |
Debt | 1,665.7 | 1,690.1 |  |
Fair Value Measurements at Reporting Date Using Significant Observable Inputs (Level 2) |  |  |  |
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 0.0 | 0.0 |  |
Derivative financial instruments: Assets | 18.0 | 7.6 |  |
Derivative financial instruments: Liabilities | 4.6 | 2.7 |  |
Debt | 298.8 | 313.1 |  |
Total Carrying Amount |  |  |  |
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 584.0 | 687.7 |  |
Derivative financial instruments: Assets | 18.0 | 7.6 |  |
Derivative financial instruments: Liabilities | 4.6 | 2.7 |  |
Debt | 1,765.2 | 1,863.7 |  |
Fair Value Measurements at Reporting Date Using Total Estimated Fair Value |  |  |  |
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items] |  |  |  |
Cash and cash equivalents | 584.0 | 687.7 |  |
Derivative financial instruments: Assets | 18.0 | 7.6 |  |
Derivative financial instruments: Liabilities | 4.6 | 2.7 |  |
Debt | 1,964.5 | 2,003.2 |  |
Fair Value Measurements at Reporting Date Using Total Estimated Fair Value | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1) | ATI Convertible Senior Notes, 3.5%, Due 2025 [Member] | Convertible Debt [Member] |  |  |  |
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items] |  |  |  |
Debt | $ 590.0 | $ 379.0 |  |

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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References No definition available. + Details
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