Business Segments - Additional Information (Details)	3 Months Ended			6 Months Ended
	Jun. 30, 2024 USD ($)	Jul. 02, 2023 USD ($)	Apr. 02, 2023 USD ($)	Jun. 30, 2024 USD ($) segment	Jul. 02, 2023 USD ($)	Dec. 31, 2023 USD ($)
Restructuring Cost and Reserve [Line Items]
Number of business segments | segment				2
Employee retention credits	$ 8,600,000			$ 8,600,000
Restructuring charges (credits)				1,700,000
Restructuring and other charges	(5,400,000)	$ (9,200,000)		(8,500,000)	$ (10,400,000)
Restructuring charges (credits)	(1,900,000)	2,700,000		(1,700,000)	2,700,000
Start Up Costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges (credits)	1,800,000	4,500,000	$ 1,200,000	4,700,000
Asset Write-offs
Restructuring Cost and Reserve [Line Items]
Restructuring charges (credits)		2,000,000.0
Facility Closing
Restructuring Cost and Reserve [Line Items]
Depreciation		$ 800,000			$ 800,000
Inventory write-offs
Restructuring Cost and Reserve [Line Items]
Restructuring charges (credits)	5,500,000			5,500,000
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		$ 291,400,000
High Performance Materials & Components
Restructuring Cost and Reserve [Line Items]
Employee retention credits	3,500,000			3,500,000
Advanced Alloys & Solutions
Restructuring Cost and Reserve [Line Items]
Employee retention credits	$ 5,100,000			$ 5,100,000