Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022	Feb. 02, 2022
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income attributable to ATI	$ 70.1	$ 30.9
Net income attributable to ATI after assumed conversions	$ 72.7	$ 34.9
Denominator for basic net income per common share – weighted average shares	128.5	126.4
Share-based compensation	2.8	1.8
Denominator for diluted net income per common share – adjusted weighted average shares and assumed conversions	150.1	152.8
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.55	$ 0.24
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.48	$ 0.23
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	0.0	0.0
Purchase of treasury stock	$ (10.1)	$ (89.9)
Stock repurchase plan, authorized amount				$ 150.0
Treasury Stock, Shares, Acquired	0.2	3.5
4.75% Convertible Senior Notes due 2022
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Convertible Senior Notes- Effect on numerator	$ 0.0	$ 1.1
Convertible Senior Notes- Effect on denominator	0.0	5.8
Interest rate		4.75%
ATI Inc. 3.5% Convertible Senior Notes due 2025
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Convertible Senior Notes- Effect on numerator	$ 2.6	$ 2.9
Convertible Senior Notes- Effect on denominator	18.8	18.8
ATI Inc. 3.5% Convertible Senior Notes due 2025 | Convertible Debt
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Interest rate	3.50%		3.50%