Debt - Revolving Credit Facility Narrative (Details) - USD ($)	9 Months Ended
	Sep. 29, 2024	Oct. 01, 2023	Sep. 09, 2027	Mar. 31, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Debt issuance costs	$ 14,800,000				$ 19,600,000
Foreign credit facilities
Debt Instrument [Line Items]
Maximum borrowing capacity	59,000,000
Short-term Debt	0				5,000,000.0
Domestic Bank Group $600 million asset-based credit facility
Debt Instrument [Line Items]
Bridge loan	60,000,000
Amount utilized to support the issuance of letters of credit	31,700,000
Average borrowings during period	0	$ 17,000,000
Short-term Debt	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
Minimum fixed charge coverage ratio allowed in event of default	1.00
Percentage of borrowing capacity unavailable due to fixed charge coverage ratio covenant	10.00%
Borrowing capacity unavailable due to fixed charge coverage ratio covenant, minimum	$ 60,000,000.0
Minimum required liquidity number of days prior to maturity of senior notes	90 days
Debt covenant, undrawn availability requirement, amount	$ 120,000,000
Debt covenant, undrawn availability requirement, percent of total available liquidity	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
2027 Term Loan
Debt Instrument [Line Items]
Outstanding borrowings	200,000,000.0				200,000,000.0
Minimum prepayment increments allowed	$ 25,000,000
Interest rate				4.21%
2027 Term Loan | Interest Rate Swap
Debt Instrument [Line Items]
Notional amount of derivative				$ 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
Interest rate	6.95%				6.95%
ATI 2030 Notes
Debt Instrument [Line Items]
Outstanding borrowings	$ 425,000,000.0				$ 425,000,000.0
Interest rate	7.25%				7.25%
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