Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Apr. 28, 2023	Dec. 31, 2022	Feb. 02, 2022
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income attributable to ATI	$ 75.7	$ 61.1	$ 221.8	$ 54.0
Net income attributable to ATI after assumed conversions	$ 78.4	$ 63.9	$ 229.7	$ 54.0
Denominator for basic net income per common share – weighted average shares	128.1	129.8	128.4	126.9
Share-based compensation	3.3	2.2	2.9	2.0
Denominator for diluted net income per common share – adjusted weighted average shares and assumed conversions	150.2	150.8	150.1	128.9
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.59	$ 0.47	$ 1.73	$ 0.43
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.52	$ 0.42	$ 1.53	$ 0.42
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	0.0	0.0	0.0	22.5
Purchase of treasury stock	$ (45.0)	$ (15.0)	$ (55.1)	$ (104.9)
Stock repurchase plan, authorized amount					$ 75.0		$ 150.0
Treasury Stock, Shares, Acquired	1.0	0.5	1.2	4.0
Purchase of treasury stock	$ (45.4)	$ (15.0)	$ (55.5)	$ (104.9)
Treasury Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Purchase of treasury stock	(45.4)	(15.0)	(55.5)	(104.9)
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	$ 2.7	$ 2.8	$ 7.9	$ 0.0
Convertible Senior Notes- Effect on denominator	18.8	18.8	18.8	0.0
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%			3.50%