Business Segments - Additional Information (Details)	3 Months Ended
	Mar. 31, 2024 USD ($) segment	Apr. 02, 2023 USD ($)	Dec. 31, 2023 USD ($)
Restructuring Cost and Reserve [Line Items]
Number of business segments | segment	2
Employee retention credits	$ 28,000,000
Restructuring charges (credits)	200,000	$ 0
Restructuring and other charges	(3,100,000)	$ (1,200,000)
Start Up Costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges (credits)	2,900,000
ATI Inc. 3.5% Convertible Senior Notes due 2025 | Convertible Debt
Restructuring Cost and Reserve [Line Items]
Debt, Gross	$ 291,400,000		$ 291,400,000