Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income (loss) attributable to ATI	$ 48.7	$ (50.1)	$ (8.4)	$ (451.6)
Net income (loss) attributable to ATI after assumed conversions	$ 52.7	$ (50.1)	$ (8.4)	$ (451.6)
Denominator for basic net income (loss) per common share – weighted average shares	127.2	126.6	127.0	126.5
Share-based compensation	0.8
Denominator for diluted net income (loss) per common share – adjusted weighted average shares and assumed conversions	152.6	126.6	127.0	126.5
Basic net (loss) income attributable to ATI per common share (in dollars per share)	$ 0.38	$ (0.40)	$ (0.07)	$ (3.57)
Diluted net (loss) income attributable to ATI per common share (in dollars per share)	$ 0.35	$ (0.40)	$ (0.07)	$ (3.57)
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	0.0	25.0	25.6	21.9
Allegheny Technologies 4.75% Convertible Senior Notes due 2022
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Convertible Senior Notes- Effect on numerator	$ 1.1
Convertible Senior Notes- Effect on denominator	5.8
Interest rate	4.75%		4.75%		4.75%
Allegheny Technologies 3.5% Convertible Senior Notes due 2025
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Convertible Senior Notes- Effect on numerator	$ 2.9
Convertible Senior Notes- Effect on denominator	18.8