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Debt - Convertible Notes Narrative (Details)
$ / shares in Units, shares in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2022
USD ($)
$ / shares
shares
Jun. 30, 2021
USD ($)
Jun. 30, 2022
USD ($)
tradingDays
$ / shares
Jun. 30, 2021
USD ($)
Dec. 31, 2021
USD ($)
$ / shares
Mar. 31, 2022
USD ($)
Mar. 31, 2021
USD ($)
Dec. 31, 2020
USD ($)
Debt Instrument [Line Items]                
Equity adjustment for adoption $ 893,900,000 $ 606,100,000 $ 893,900,000 $ 606,100,000 $ 832,700,000 $ 795,700,000 $ 609,300,000 $ 641,400,000
Debt issuance costs 18,900,000   18,900,000   20,800,000      
Interest expense, net $ (23,400,000) $ (23,700,000) (47,000,000.0) (47,100,000)        
Stock Issued During Period, Shares, Conversion of Convertible Securities | shares 5.7              
Repayment of Long-Term Debt, Long-Term Lease Obligation, and Capital Security     11,600,000 $ 7,100,000        
Conversion of convertible notes $ 82,500,000   82,500,000          
Estimate Of Fair Value Fair Value Disclosure                
Debt Instrument [Line Items]                
Debt 1,761,800,000   1,761,800,000   2,003,200,000      
Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure                
Debt Instrument [Line Items]                
Debt 1,473,400,000   1,473,400,000   1,690,100,000      
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%   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 $ 5,600,000   $ 5,600,000   $ 6,500,000      
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 $ 2,500,000 $ 5,100,000 $ 5,100,000        
Amortization of Debt Issuance Costs 500,000 400,000 900,000 800,000        
Interest expense, net 3,100,000 $ 2,900,000 $ 6,000,000.0 $ 5,900,000        
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 467,000,000   $ 467,000,000   379,000,000      
ATI Inc. 4.75% Convertible Senior Notes due 2022                
Debt Instrument [Line Items]                
Debt, Gross 0   $ 0   $ 84,200,000      
Interest rate         4.75%      
Repayment of Long-Term Debt, Long-Term Lease Obligation, and Capital Security $ 1,700,000              
ATI Inc. 4.75% Convertible Senior Notes due 2022 | Convertible Debt                
Debt Instrument [Line Items]                
Debt instrument, convertible, conversion ratio         0.0692042      
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares         $ 14.45      
Debt issuance costs         $ 300,000      
Debt instrument, interest rate, effective percentage 5.40% 5.40% 5.40% 5.40%        
Interest Expense, Debt, Excluding Amortization $ 1,000,000.0 $ 1,000,000.0 $ 2,000,000.0 $ 2,000,000.0        
Amortization of Debt Issuance Costs 200,000 100,000 300,000 200,000        
Interest expense, net $ 1,200,000 $ 1,100,000 $ 2,300,000 $ 2,200,000        
ATI Inc. 4.75% Convertible Senior Notes due 2022 | Convertible Debt | Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure                
Debt Instrument [Line Items]                
Debt         $ 102,000,000