Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Feb. 02, 2022	Dec. 31, 2021
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net loss attributable to ATI	$ (38.0)	$ (49.2)	$ (7.1)	$ (57.1)
Net loss attributable to ATI after assumed conversions	$ (38.0)	$ (49.2)	$ (7.1)	$ (57.1)
Denominator for basic net loss per common share – weighted average shares	124.6	127.1	125.5	127.0
Share-based compensation	0.0	0.0	0.0	0.0
Denominator for diluted net loss per common share – adjusted weighted average shares and assumed conversions	124.6	127.1	125.5	127.0
Basic net loss attributable to ATI per common share (in dollars per share)	$ (0.31)	$ (0.39)	$ (0.06)	$ (0.45)
Diluted net loss attributable to ATI per common share (in dollars per share)	$ (0.31)	$ (0.39)	$ (0.06)	$ (0.45)
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	26.1	25.7	26.2	25.7
Purchase of treasury stock			$ (89.9)	$ 0.0
Stock repurchase plan, authorized amount					$ 150.0
Treasury Stock, Shares, Acquired			3.5
ATI Inc. 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	$ 0.0	$ 0.0	$ 0.0
Convertible Senior Notes- Effect on denominator	0.0	0.0	0.0	0.0
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	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Convertible Senior Notes- Effect on denominator	0.0	0.0	0.0	0.0