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Debt - Revolving Credit Facility Narrative (Details)
 3 Months Ended 9 Months Ended
Aug. 11, 2023
USD ($)
Sep. 30, 2023
USD ($)
Sep. 30, 2023
USD ($)
segment
Sep. 30, 2022
USD ($)
Sep. 09, 2027
USD ($)
Dec. 31, 2022
USD ($)
Debt Instrument [Line Items]            
Debt issuance costs   $ 20,700,000 $ 20,700,000     $ 17,200,000
Pension Plans Defined Benefit            
Debt Instrument [Line Items]            
Contributions by employer   222,000,000 272,000,000      
Foreign credit facilities            
Debt Instrument [Line Items]            
Maximum borrowing capacity   57,000,000 57,000,000      
Short-term Debt   11,200,000 11,200,000     19,400,000
Domestic Bank Group $600 million asset-based credit facility            
Debt Instrument [Line Items]            
Bridge loan   60,000,000 60,000,000      
Amount utilized to support the issuance of letters of credit   31,700,000 31,700,000      
Average borrowings during period     17,000,000 $ 0    
Short-term Debt   0 $ 0     0
Interest rate during period     6.50%      
Domestic Bank Group $600 million asset-based credit facility | Revolving credit facility            
Debt Instrument [Line Items]            
Maximum borrowing capacity   $ 600,000,000 $ 600,000,000      
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 $ 60,000,000      
Minimum required liquidity number of days prior to maturity of senior notes     90 days      
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%      
Domestic Bank Group $600 million asset-based credit facility | Revolving credit facility | Minimum | Base rate borrowings            
Debt Instrument [Line Items]            
Variable interest rate (percentage)     0.25%      
Domestic Bank Group $600 million asset-based credit facility | Revolving credit facility | Minimum | Secured Overnight Financing Rate (SOFR)            
Debt Instrument [Line Items]            
Variable interest rate (percentage)     1.25%      
Domestic Bank Group $600 million asset-based credit facility | Revolving credit facility | Maximum | Base rate borrowings            
Debt Instrument [Line Items]            
Variable interest rate (percentage)     0.75%      
Domestic Bank Group $600 million asset-based credit facility | Revolving credit facility | Maximum | Secured Overnight Financing Rate (SOFR)            
Debt Instrument [Line Items]            
Variable interest rate (percentage)     1.75%      
Domestic Bank Group $600 million asset-based credit facility | Letter of credit sub-facility            
Debt Instrument [Line Items]            
Maximum borrowing capacity   $ 200,000,000 $ 200,000,000      
2027 Term Loan            
Debt Instrument [Line Items]            
Outstanding borrowings   $ 200,000,000.0 200,000,000.0     200,000,000.0
Minimum prepayment increments allowed     $ 25,000,000      
Interest rate   4.21% 4.21%      
2027 Term Loan | Interest Rate Swap            
Debt Instrument [Line Items]            
Notional amount of derivative   $ 50,000,000 $ 50,000,000      
2027 Term Loan | Secured Overnight Financing Rate (SOFR)            
Debt Instrument [Line Items]            
Variable interest rate (percentage)     2.00%      
Allegheny Ludlum 6.95% Debentures due 2025 (a)            
Debt Instrument [Line Items]            
Outstanding borrowings   $ 150,000,000.0 $ 150,000,000.0     $ 150,000,000.0
Interest rate   6.95% 6.95%     6.95%
ATI 2030 Notes            
Debt Instrument [Line Items]            
Outstanding borrowings   $ 425,000,000.0 $ 425,000,000.0     $ 0
Interest rate   7.25% 7.25%      
Debt issuance costs $ 6,200,000          
Debt instrument, amortization period     7 years      
Debt instrument, redemption price, percentage     101.00%      
Proceeds from Debt, Net of Issuance Costs $ 418,800,000          
ATI 2030 Notes | Minimum            
Debt Instrument [Line Items]            
Debt Instrument, Notice Period Required for Redemption | segment     15      
ATI 2030 Notes | Maximum            
Debt Instrument [Line Items]            
Debt Instrument, Notice Period Required for Redemption | segment     60      
Forecast | Domestic Bank Group $600 million asset-based credit facility | Revolving credit facility            
Debt Instrument [Line Items]            
Line of credit facility, increase to maximum borrowing capacity         $ 300,000,000