Debt - Convertible Notes Narrative (Details)	3 Months Ended
	Mar. 31, 2024 USD ($) tradingDays $ / shares	Apr. 02, 2023 USD ($)	Dec. 31, 2023 USD ($)	Jan. 01, 2023 USD ($)
Debt Instrument [Line Items]
Equity adjustment for adoption	$ 1,373,200,000	$ 1,214,300,000	$ 1,480,500,000	$ 1,157,200,000
Debt issuance costs	18,500,000		19,600,000
Interest expense, net	26,600,000	$ 19,900,000
Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt	2,836,800,000		2,746,700,000
Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt	2,536,200,000		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
Interest rate	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
Debt issuance costs	$ 2,400,000		$ 2,900,000
Debt instrument, interest rate, effective percentage	4.20%	4.20%
Cap price (in dollars per share) | $ / shares	$ 19.76
Interest Expense, Debt, Excluding Amortization	$ 2,500,000	$ 2,500,000
Amortization of Debt Issuance Costs	500,000	500,000
Interest expense, net	$ 3,000,000.0	$ 3,000,000.0
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	$ 967,000,000		$ 864,000,000