Debt - Narrative (Details) $ / shares in Units, shares in Millions		3 Months Ended			8 Months Ended	12 Months Ended
	Aug. 11, 2023 USD ($)	Sep. 29, 2024 USD ($) $ / shares shares	Sep. 10, 2024 USD ($)	Oct. 01, 2023 USD ($)	Sep. 10, 2024	Dec. 29, 2024 USD ($)	Dec. 31, 2023 USD ($)	Jan. 01, 2023 USD ($) shares	Jan. 02, 2022 $ / shares	Sep. 09, 2027 USD ($)	Mar. 31, 2024 USD ($)	Sep. 09, 2022 USD ($)	Sep. 08, 2022 USD ($)
Debt Instrument [Line Items]
Payments of debt issuance costs						$ 0	$ 6,200,000	$ 0
Debt Issuance Costs, Net						14,200,000	19,600,000
Total interest expense						108,200,000	92,800,000	87,400,000
Conversion of convertible notes		$ 291,000,000.0				291,700,000		$ 82,500,000
Stock Issued During Period, Shares, Conversion of Convertible Securities | shares		18.8						5.7
Repayment of Long-term Debt, Long-term Lease Obligation, and Capital Security						29,600,000	25,200,000	$ 23,100,000
Interest expense						124,200,000	105,800,000	92,100,000
Interest costs capitalized						11,800,000	13,500,000	5,100,000
Interest costs paid						131,400,000	114,700,000	92,800,000
Interest income						16,000,000.0	13,000,000.0	4,700,000
2025						180,500,000
2026						25,000,000.0
2027						572,200,000
2028						16,700,000
2029						335,800,000
Receipt of convertible note capped call		$ 76,100,000				76,100,000	0	$ 0
Pension Benefits [Member]
Debt Instrument [Line Items]
Employer contributions				$ 222,000,000		3,300,000	278,000,000.0
Pension plan contributions				$ 222,000,000		3,300,000	278,000,000.0
Fair Value Measurements at Reporting Date Using Total Estimated Fair Value
Debt Instrument [Line Items]
Debt						1,889,700,000	2,746,700,000
Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1)
Debt Instrument [Line Items]
Debt						1,580,200,000	2,438,900,000
Foreign credit agreements [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount						0	5,000,000.0
ATI Inc., Convertible Senior Notes, 3.5%, Due 2025
Debt Instrument [Line Items]
Repayment of Long-term Debt, Long-term Lease Obligation, and Capital Security		400,000
ATI Inc., Convertible Senior Notes, 3.5%, Due 2025 | Convertible Debt [Member]
Debt Instrument [Line Items]
Write off of Deferred Debt Issuance Cost		$ 1,600,000
Debt Instrument, Convertible, Conversion Price with Premium | $ / shares		$ 15.45
Debt Instrument, Increase, Accrued Interest			$ 2,300,000
Debt Instrument, Convertible, Conversion Ratio, Premium					0.647178
ATI Convertible Senior Notes, 4.75%, Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage									4.75%
ATI Convertible Senior Notes, 4.75%, Due 2022 [Member] | Convertible Debt [Member]
Debt Instrument [Line Items]
Conversion price of convertible notes | $ / shares									$ 14.45
Debt Instrument, Interest Rate, Effective Percentage								5.40%
Interest Expense, Debt, Excluding Amortization								$ 2,000,000.0
Amortization of Debt Issuance Costs								300,000
Total interest expense								$ 2,300,000
Convertible debt conversion rate in shares of stock									0.0692042
ATI $350 million 5.875% Senior Notes due 2027
Debt Instrument [Line Items]
Debt instrument carrying amount						$ 350,000,000.0	$ 350,000,000.0
Debt Instrument, Issuer						ATI Inc.	ATI Inc.
Debt Instrument, Maturity Date						Dec. 01, 2027	Dec. 01, 2027
ATI $350 million 5.875% Senior Notes due 2027 | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1)
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage						5.875%	5.875%
ATI Convertible Senior Notes, 3.5%, Due 2025 [Member] | Convertible Debt [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage							3.50%
Debt instrument carrying amount						$ 0	$ 291,400,000
Conversion price of convertible notes | $ / shares		15.49
Debt Issuance Costs, Net							$ 2,900,000
Debt Instrument, Interest Rate, Effective Percentage						4.20%	4.20%	4.20%
Debt Instrument, Convertible, Cap Price | $ / shares		$ 19.76
Interest Expense, Debt, Excluding Amortization						$ 7,200,000	$ 10,200,000	$ 10,200,000
Amortization of Debt Issuance Costs						1,300,000	1,900,000	1,800,000
Total interest expense						8,500,000	$ 12,100,000	12,000,000.0
Convertible debt conversion rate in shares of stock							0.0645745
Debt Instrument, Issuer							ATI Inc.
Debt Instrument, Maturity Date							Jun. 15, 2025
ATI Convertible Senior Notes, 3.5%, Due 2025 [Member] | Convertible Debt [Member] | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measurements at Reporting Date Using Total Estimated Fair Value
Debt Instrument [Line Items]
Debt							$ 864,000,000
ATI 2029 Notes
Debt Instrument [Line Items]
Debt instrument carrying amount						$ 325,000,000.0	$ 325,000,000.0
Debt Instrument, Issuer						ATI Inc.	ATI Inc.
Debt Instrument, Maturity Date						Oct. 01, 2029	Oct. 01, 2029
ATI 2029 Notes | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1)
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage						4.875%	4.875%
ATI 2031Notes
Debt Instrument [Line Items]
Debt instrument carrying amount						$ 350,000,000.0	$ 350,000,000.0
Debt Instrument, Issuer						ATI Inc.	ATI Inc.
Debt Instrument, Maturity Date						Oct. 01, 2031	Oct. 01, 2031
ATI 2031Notes | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1)
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage						5.125%	5.125%
ATI Inc., Convertible Senior Notes, 4.75%, Due 2022
Debt Instrument [Line Items]
Repayment of Long-term Debt, Long-term Lease Obligation, and Capital Security								$ 1,700,000
Domestic Bank Group $600 million asset-based credit facility
Debt Instrument [Line Items]
Line of credit facility amount outstanding						$ 30,500,000
Average outstanding amount						0	$ 13,000,000
Bridge Loan						60,000,000
Short-Term Debt						0	$ 0
Interest rate during period							6.50%
2027 Term Loan
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage											4.21%
Debt instrument carrying amount						200,000,000	$ 200,000,000.0
Debt Instrument, Prepayment Increments, Minimum						$ 25,000,000
Debt Instrument, Issuer						ATI Inc.	ATI Inc.
Debt Instrument, Maturity Date						Sep. 30, 2027	Sep. 30, 2027
2027 Term Loan | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Variable rate spread						2.00%
ATI 2030 Notes
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage						7.25%	7.25%
Debt instrument carrying amount						$ 425,000,000.0	$ 425,000,000.0
Debt Issuance Costs, Net	$ 6,200,000
Debt instrument, term						7 years
Debt instrument, redemption price, percentage						101.00%
Proceeds from Debt, Net of Issuance Costs	$ 418,800,000
Debt Instrument, Issuer						ATI Inc.	ATI Inc.
Debt Instrument, Maturity Date						Aug. 15, 2030	Aug. 15, 2030
ATI 2030 Notes | Minimum
Debt Instrument [Line Items]
Debt instrument, notice period required for redemption						15 days
ATI 2030 Notes | Maximum
Debt Instrument [Line Items]
Debt instrument, notice period required for redemption						60 days
Allegheny Ludlum 6.95% Debentures due 2025 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage						6.95%	6.95%
Debt instrument carrying amount						$ 150,000,000.0	$ 150,000,000.0
Debt Instrument, Issuer						Allegheny Ludlum	Allegheny Ludlum
Debt Instrument, Maturity Date						Dec. 15, 2025	Dec. 15, 2025
Allegheny Ludlum 6.95% Debentures due 2025 [Member] | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1)
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage						6.95%	6.95%
Revolving Credit Facility [Member] | Domestic Bank Group $600 million asset-based credit facility
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity						$ 600,000,000
Fixed charge coverage ratio						1.00
Remaining borrowing capacity, percent of minimum borrowing capacity						10.00%
Minimum remaining borrowing capacity						$ 60,000,000.0
Debt Issuance Costs, Net												$ 2,400,000	$ 1,700,000
Debt Covenant, Undrawn Availability Requirement, Amount						$ 120,000,000
Debt Covenant, Undrawn Availability Requirement, Percent Of Total Available Liquidity						20.00%
Revolving Credit Facility [Member] | Domestic Bank Group $600 million asset-based credit facility | Minimum | Base Rate [Member]
Debt Instrument [Line Items]
Variable rate spread						0.25%
Revolving Credit Facility [Member] | Domestic Bank Group $600 million asset-based credit facility | Minimum | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Variable rate spread						1.25%
Revolving Credit Facility [Member] | Domestic Bank Group $600 million asset-based credit facility | Maximum | Base Rate [Member]
Debt Instrument [Line Items]
Variable rate spread						0.75%
Revolving Credit Facility [Member] | Domestic Bank Group $600 million asset-based credit facility | Maximum | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Variable rate spread						1.75%
Letter of Credit [Member] | Domestic Bank Group $600 million asset-based credit facility
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity						$ 200,000,000
Foreign Line of Credit
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity						$ 70,000,000
Interest Rate Swap [Member] | 2027 Term Loan
Debt Instrument [Line Items]
Derivative notional amount											$ 50,000,000
Forecast | Revolving Credit Facility [Member] | Domestic Bank Group $600 million asset-based credit facility
Debt Instrument [Line Items]
Line of Credit Facility, Increase to Maximum Borrowing Capacity										$ 300,000,000