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Debt - Convertible Notes Narrative (Details)
 3 Months Ended 6 Months Ended
Jun. 30, 2023
USD ($)
$ / shares
Jun. 30, 2022
USD ($)
Jun. 30, 2023
USD ($)
tradingDays
$ / shares
Jun. 30, 2022
USD ($)
Mar. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
Mar. 31, 2022
USD ($)
Dec. 31, 2021
USD ($)
Debt Instrument [Line Items]                
Equity adjustment for adoption $ 1,309,200,000 $ 893,900,000 $ 1,309,200,000 $ 893,900,000 $ 1,214,300,000 $ 1,157,200,000 $ 795,700,000 $ 832,700,000
Debt issuance costs 15,500,000   15,500,000     17,200,000    
Interest expense, net 21,300,000 $ 23,400,000 41,200,000 $ 47,000,000.0        
Estimate Of Fair Value Fair Value Disclosure                
Debt Instrument [Line Items]                
Debt 2,267,300,000   2,267,300,000     1,964,500,000    
Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure                
Debt Instrument [Line Items]                
Debt 1,944,400,000   1,944,400,000     1,665,700,000    
ATI Inc. 3.5% Convertible Senior Notes due 2025 | Convertible Debt                
Debt Instrument [Line Items]                
Debt, Gross $ 291,400,000   $ 291,400,000     291,400,000    
Interest rate 3.50%   3.50%          
Debt instrument, convertible, redemption price as a percent of principal amount     100.00%          
Debt instrument, convertible, threshold percentage of stock price trigger     130.00%          
Debt instrument, convertible, threshold trading days | tradingDays     20          
Debt instrument, convertible, threshold consecutive trading days | tradingDays     30          
Debt instrument, convertible, conversion ratio     0.0645745          
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares $ 15.49   $ 15.49          
Debt issuance costs $ 3,800,000   $ 3,800,000     4,800,000    
Debt instrument, interest rate, effective percentage 4.20% 4.20% 4.20% 4.20%        
Cap price (in dollars per share) | $ / shares $ 19.76   $ 19.76          
Interest Expense, Debt, Excluding Amortization $ 2,600,000 $ 2,600,000 $ 5,100,000 $ 5,100,000        
Amortization of Debt Issuance Costs 400,000 500,000 900,000 900,000        
Interest expense, net 3,000,000.0 $ 3,100,000 $ 6,000,000.0 $ 6,000,000.0        
Debt instrument, convertible, number of equity instruments (in shares)     18,800,000          
ATI Inc. 3.5% Convertible Senior Notes due 2025 | Convertible Debt | Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure                
Debt Instrument [Line Items]                
Debt $ 844,000,000   $ 844,000,000     $ 590,000,000