Business Segments - Additional Information (Details)	3 Months Ended				9 Months Ended
	Sep. 30, 2022 USD ($)	Jun. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($) segment	Sep. 30, 2021 USD ($)	Dec. 31, 2021 USD ($)
Restructuring Cost and Reserve [Line Items]
Number of business segments | segment					2
Government sponsored COVID relief income					$ 34,300,000
Gain (Loss) on sale of business	$ 0			$ 0	(134,200,000)	$ 0
Litigation Reserve	19,900,000			0	28,500,000	0
Restructuring charges (credits)	(2,600,000)			(2,300,000)	(5,000,000.0)	(8,500,000)
Strike related costs	0			(22,900,000)	0	(63,200,000)
Gain (loss) on asset sales and sale of businesses, net	0			13,700,000	(134,200,000)	13,700,000
Employee Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges (credits)						(9,200,000)
Facility idling
Restructuring Cost and Reserve [Line Items]
Restructuring charges (credits)						700,000
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
Sheffield, UK Operations
Restructuring Cost and Reserve [Line Items]
Gain (Loss) on sale of business		$ (115,900,000)	$ (25,100,000)		(141,000,000)
Pico Rivera, CA Operations
Restructuring Cost and Reserve [Line Items]
Gain (Loss) on sale of assets					6,800,000
High Performance Materials & Components
Restructuring Cost and Reserve [Line Items]
Employee retention credits and Aviation Manufacturing Jobs Protection Program					27,500,000
High Performance Materials & Components | Operating Segments
Restructuring Cost and Reserve [Line Items]
Strike related costs				(1,400,000)		(3,500,000)
Advanced Alloys & Solutions
Restructuring Cost and Reserve [Line Items]
Employee retention credits					$ 6,800,000
Advanced Alloys & Solutions | Operating Segments
Restructuring Cost and Reserve [Line Items]
Strike related costs				$ (21,500,000)		$ (59,700,000)