Debt - Convertible Notes Narrative (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2024 USD ($) $ / shares	Jul. 02, 2023 USD ($)	Jun. 30, 2024 USD ($) tradingDays $ / shares	Jul. 02, 2023 USD ($)	Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Apr. 02, 2023 USD ($)	Jan. 01, 2023 USD ($)
Debt Instrument [Line Items]
Equity adjustment for adoption	$ 1,467,900,000	$ 1,309,200,000	$ 1,467,900,000	$ 1,309,200,000	$ 1,373,200,000	$ 1,480,500,000	$ 1,214,300,000	$ 1,157,200,000
Debt issuance costs	17,400,000		17,400,000			19,600,000
Interest expense, net	28,400,000	$ 21,300,000	55,000,000.0	$ 41,200,000
Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt	2,905,700,000		2,905,700,000			2,746,700,000
Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt	2,608,900,000		2,608,900,000			2,438,900,000
ATI Inc. 3.5% Convertible Senior Notes due 2025 | Convertible Debt
Debt Instrument [Line Items]
Debt, Gross	$ 291,400,000		$ 291,400,000			$ 291,400,000
Interest rate	3.50%		3.50%			3.50%
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 instrument, convertible, conversion ratio			0.0645745
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares	$ 15.49		$ 15.49
Debt issuance costs	$ 2,000,000.0		$ 2,000,000.0			$ 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	$ 2,600,000	$ 2,600,000	$ 5,100,000	$ 5,100,000
Amortization of Debt Issuance Costs	500,000	400,000	1,000,000.0	900,000
Interest expense, net	3,100,000	$ 3,000,000.0	$ 6,100,000	$ 6,000,000.0
Debt instrument, convertible, number of equity instruments (in shares)			18,800,000
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	$ 1,040,000,000.00		$ 1,040,000,000.00			$ 864,000,000