v3.26.1
Debt - Narrative (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 03, 2026
Jun. 28, 2026
Jun. 28, 2026
Jun. 08, 2026
Dec. 28, 2025
Jun. 29, 2025
Debt Instrument [Line Items]            
Debt issuance costs   $ 17,100,000 $ 17,100,000   $ 11,600,000  
ATI 2033 Notes | Finance leases and other            
Debt Instrument [Line Items]            
Aggregate principal amount $ 450,000,000          
Interest rate 5.875%          
Proceeds from debt, net of issuance costs $ 443,100,000          
Debt issuance costs $ 6,900,000          
Debt instrument, term 7 years          
Debt instrument, redemption price, percentage 101.00%          
Debt instrument carrying amount   $ 450,000,000.0 $ 450,000,000.0   0  
ATI Inc. 5.875% Senior Notes due 2027 | Finance leases and other            
Debt Instrument [Line Items]            
Interest rate   5.875% 5.875% 5.875%    
Debt instrument carrying amount   $ 350,000,000.0 $ 350,000,000.0 $ 350,000,000 350,000,000.0  
Credit Facility            
Debt Instrument [Line Items]            
Amount utilized to support the issuance of letters of credit   29,300,000 29,300,000      
Average borrowings during period   $ 65,900,000 $ 64,000,000.0      
Average annual interest rate   5.00% 5.20%      
Credit Facility | Line of Credit            
Debt Instrument [Line Items]            
Debt issuance costs   $ 2,800,000 $ 2,800,000      
Minimum fixed charge coverage ratio allowed in event of default   1.00 1.00      
Percentage of borrowing capacity unavailable due to fixed charge coverage ratio covenant   10.00% 10.00%      
Borrowing capacity unavailable due to fixed charge coverage ratio covenant, minimum   $ 60,000,000.0 $ 60,000,000.0      
Minimum required liquidity number of days prior to maturity of senior notes     90 days      
Unamortized debt issuance expense   1,900,000 $ 1,900,000      
Credit Facility | Line of Credit | Minimum | Secured Overnight Financing Rate (SOFR)            
Debt Instrument [Line Items]            
Variable interest rate (percentage)     1.25%      
Credit Facility | Line of Credit | Minimum | Base rate borrowings            
Debt Instrument [Line Items]            
Variable interest rate (percentage)     0.25%      
Credit Facility | Line of Credit | Maximum | Secured Overnight Financing Rate (SOFR)            
Debt Instrument [Line Items]            
Variable interest rate (percentage)     1.75%      
Credit Facility | Line of Credit | Maximum | Base rate borrowings            
Debt Instrument [Line Items]            
Variable interest rate (percentage)     0.75%      
Credit Facility | Line of Credit | Revolving credit facility            
Debt Instrument [Line Items]            
Debt instrument carrying amount   0 $ 0   0 $ 0
Maximum borrowing capacity   600,000,000 600,000,000      
Line of credit facility, increase to maximum borrowing capacity   300,000,000 300,000,000      
Debt covenant, undrawn availability requirement, amount   $ 120,000,000 $ 120,000,000      
Debt covenant, undrawn availability requirement, percent of total available liquidity   20.00% 20.00%      
Credit Facility | Line of Credit | Letter of credit sub-facility            
Debt Instrument [Line Items]            
Maximum borrowing capacity   $ 200,000,000 $ 200,000,000      
Credit Facility | Line of Credit | Secured Debt            
Debt Instrument [Line Items]            
Debt instrument carrying amount   200,000,000.0 200,000,000.0   200,000,000.0  
Maximum borrowing capacity   200,000,000 200,000,000      
Line of credit facility, accordion feature, increase limit   100,000,000 $ 100,000,000      
Variable interest rate (percentage)     2.00%      
Minimum prepayment increments allowed     $ 25,000,000      
Credit Facility | Line of Credit | Bridge Loan            
Debt Instrument [Line Items]            
Maximum borrowing capacity   $ 60,000,000 $ 60,000,000      
ATI Inc. 4.875% Senior Notes due 2029 | Finance leases and other            
Debt Instrument [Line Items]            
Interest rate   4.875% 4.875%      
Debt instrument carrying amount   $ 325,000,000.0 $ 325,000,000.0   325,000,000.0  
Foreign Credit Agreements | Foreign credit facilities            
Debt Instrument [Line Items]            
Maximum borrowing capacity   75,400,000 75,400,000      
Foreign Credit Agreements | Line of Credit | Foreign credit facilities            
Debt Instrument [Line Items]            
Debt instrument carrying amount   $ 2,700,000 $ 2,700,000   $ 0