Debt - Convertible Notes Narrative (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($) $ / shares	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) tradingDays $ / shares	Sep. 30, 2022 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)	Jun. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Debt Instrument [Line Items]
Equity adjustment for adoption	$ 1,355,900,000	$ 938,400,000	$ 1,355,900,000	$ 938,400,000	$ 1,309,200,000	$ 1,157,200,000	$ 893,900,000	$ 832,700,000
Debt issuance costs	20,700,000		20,700,000			17,200,000
Interest expense, net	23,800,000	$ 20,800,000	65,000,000.0	$ 67,800,000
Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt	2,359,300,000		2,359,300,000			1,964,500,000
Fair Value Inputs Level 1 | Estimate Of Fair Value Fair Value Disclosure
Debt Instrument [Line Items]
Debt	2,044,800,000		2,044,800,000			1,665,700,000
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	$ 3,400,000		$ 3,400,000			$ 4,800,000
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	$ 2,500,000	$ 2,500,000	$ 7,600,000	$ 7,600,000
Amortization of Debt Issuance Costs	500,000	400,000	1,400,000	1,300,000
Interest expense, net	3,000,000.0	$ 2,900,000	$ 9,000,000.0	$ 8,900,000
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	$ 786,000,000		$ 786,000,000			$ 590,000,000