Debt - Convertible Notes Narrative (Details) $ / shares in Units, shares in Millions	3 Months Ended			8 Months Ended	9 Months Ended
	Sep. 29, 2024 USD ($) $ / shares shares	Sep. 10, 2024 USD ($)	Oct. 01, 2023 USD ($)	Sep. 10, 2024	Sep. 29, 2024 USD ($) $ / shares	Oct. 01, 2023 USD ($)	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)	Jul. 02, 2023 USD ($)	Jan. 01, 2023 USD ($)
Debt Instrument [Line Items]
Equity adjustment for adoption	$ 1,912,000,000		$ 1,355,900,000		$ 1,912,000,000	$ 1,355,900,000	$ 1,467,900,000	$ 1,480,500,000	$ 1,309,200,000	$ 1,157,200,000
Debt issuance costs	14,800,000				14,800,000			19,600,000
Interest expense, net	28,000,000.0		$ 23,800,000		83,000,000.0	65,000,000.0
Stock Issued During Period, Value, Conversion of Convertible Securities	$ 291,000,000.0
Stock Issued During Period, Shares, Conversion of Convertible Securities | shares	18.8
Repayment of Long-Term Debt, Long-Term Lease Obligation, and Capital Security					21,900,000	22,000,000.0
Receipt of convertible note capped call	$ 76,100,000				76,100,000	$ 0
Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt	1,911,200,000				1,911,200,000			2,746,700,000
Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt	1,613,000,000				1,613,000,000			2,438,900,000
ATI Inc. 3.5% Convertible Senior Notes due 2025
Debt Instrument [Line Items]
Repayment of Long-Term Debt, Long-Term Lease Obligation, and Capital Security	400,000
ATI Inc. 3.5% Convertible Senior Notes due 2025 | Convertible Debt
Debt Instrument [Line Items]
Debt, Gross	$ 0				$ 0			$ 291,400,000
Interest rate								3.50%
Debt instrument, convertible, conversion ratio				0.0645745
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares	$ 15.49				$ 15.49
Debt issuance costs								$ 2,900,000
Debt instrument, interest rate, effective percentage	4.20%		4.20%		4.20%	4.20%
Cap price (in dollars per share) | $ / shares	$ 19.76				$ 19.76
Interest Expense, Debt, Excluding Amortization	$ 2,100,000		$ 2,500,000		$ 7,200,000	$ 7,600,000
Amortization of Debt Issuance Costs	300,000		500,000		1,300,000	1,400,000
Interest expense, net	2,400,000		$ 3,000,000.0		$ 8,500,000	$ 9,000,000.0
Write off of deferred debt issue costs	$ (1,600,000)
Debt Instrument, Increase, Accrued Interest		$ 2,300,000
Debt Instrument, Convertible, Conversion Price with Premium | $ / shares	$ 15.45				$ 15.45
Debt Instrument, Convertible, Conversion Ratio, Premium				0.0647178
ATI Inc. 3.5% Convertible Senior Notes due 2025 | Convertible Debt | Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt								$ 864,000,000