Debt - Narrative (Details) $ / shares in Units, shares in Millions		3 Months Ended	12 Months Ended
	Aug. 11, 2023 USD ($)	Oct. 01, 2023 USD ($)	Dec. 31, 2023 USD ($) tradingDays $ / shares	Jan. 01, 2023 USD ($) shares	Jan. 02, 2022 USD ($) $ / shares	Sep. 09, 2027 USD ($)	Sep. 09, 2022 USD ($)	Sep. 08, 2022 USD ($)	Jan. 03, 2021 USD ($)	Dec. 31, 2013
Debt Instrument [Line Items]
Payments of debt issuance costs			$ 6,200,000	$ 0	$ 9,500,000
Debt extinguishment charge			0	0	65,500,000
Payment for Debt Extinguishment or Debt Prepayment Cost			0	0	64,500,000
Debt Issuance Costs, Net			19,600,000	17,200,000
Interest expense, net			92,800,000	87,400,000	96,900,000
Percentage of Forecast [Abstract]
Conversion of convertible notes				$ 82,500,000
Stock Issued During Period, Shares, Conversion of Convertible Securities | shares				5.7
Repayment of Long-term Debt, Long-term Lease Obligation, and Capital Security			25,200,000	$ 23,100,000	515,600,000
Interest expense			105,800,000	92,100,000	97,600,000
Interest costs capitalized			13,500,000	5,100,000	4,300,000
Interest costs paid			114,700,000	92,800,000	97,500,000
Interest income			13,000,000	4,700,000	700,000
2024			31,900,000
2025			465,300,000
2026			18,500,000
2027			565,300,000
2028			9,600,000
Debt extinguishment charge			0	0	$ (65,500,000)
Pension Benefits [Member]
Debt Instrument [Line Items]
Employer contributions		$ 222,000,000	278,000,000.0	57,400,000
Percentage of Forecast [Abstract]
Pension plan contributions		$ 222,000,000	278,000,000.0	57,400,000
Fair Value Measurements at Reporting Date Using Total Estimated Fair Value
Debt Instrument [Line Items]
Debt			2,746,700,000	1,964,500,000
Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1)
Debt Instrument [Line Items]
Debt			2,438,900,000	1,665,700,000
Foreign credit agreements [Member]
Debt Instrument [Line Items]
Debt instrument carrying amount			$ 5,000,000.0	$ 19,400,000
ATI $500 million 5.875% Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage										5.875%
Increase in interest expense per downgrade notch, percent					0.25%
Maximum number of downgrade notches					4
Maximum increase in interest rate expense, percent					2.00%
Debt instrument carrying amount									$ 500,000,000
Write off of Deferred Debt Issuance Cost					$ 1,000,000
ATI $500 million 5.875% Senior Notes due 2023 [Member] | Maximum
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage					7.875%
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%	5.40%
Interest Expense, Debt, Excluding Amortization				$ 2,000,000.0	$ 4,000,000.0
Amortization of Debt Issuance Costs				300,000	500,000
Interest expense, net				$ 2,300,000	$ 4,500,000
Percentage of Forecast [Abstract]
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 interest rate stated percentage			5.875%	5.875%
Debt instrument carrying amount			$ 350,000,000.0	$ 350,000,000.0
ATI Convertible Senior Notes, 3.5%, Due 2025 [Member] | Convertible Debt [Member]
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage			3.50%	3.50%	3.50%
Debt instrument carrying amount			$ 291,400,000	$ 291,400,000
Number of equity instruments issuable			18,800,000
Conversion price of convertible notes | $ / shares			$ 15.49
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 Issuance Costs, Net			$ 2,900,000	$ 4,800,000
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			$ 10,200,000	$ 10,200,000	$ 10,200,000
Amortization of Debt Issuance Costs			1,900,000	1,800,000	1,700,000
Interest expense, net			$ 12,100,000	12,000,000.0	11,900,000
Percentage of Forecast [Abstract]
Convertible debt conversion rate in shares of stock			0.0645745
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	$ 590,000,000
ATI 2029 and 2031 Notes
Debt Instrument [Line Items]
Debt instrument, redemption price, percentage			101.00%
Proceeds from Debt, Net of Issuance Costs					665,700,000
ATI 2029 and 2031 Notes | Minimum
Debt Instrument [Line Items]
Debt instrument, notice period required for redemption			15 days
ATI 2029 and 2031 Notes | Maximum
Debt Instrument [Line Items]
Debt instrument, notice period required for redemption			60 days
ATI 2029 Notes
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage			4.875%	4.875%
Payments of debt issuance costs					4,700,000
Debt instrument carrying amount			$ 325,000,000.0	$ 325,000,000.0
Debt instrument, term			8 years
ATI 2031Notes
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage			5.125%	5.125%
Payments of debt issuance costs					$ 4,700,000
Debt instrument carrying amount			$ 350,000,000	$ 350,000,000.0
Debt instrument, term			10 years
ATI Inc., Convertible Senior Notes, 4.75%, Due 2022
Percentage of Forecast [Abstract]
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			$ 31,700,000
Average outstanding amount			13,000,000	0
Percentage of Forecast [Abstract]
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
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%
Debt instrument carrying amount			$ 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
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
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%
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
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			58,000,000
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