Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net loss attributable to ATI	$ (49.2)	$ (422.6)	$ (57.1)	$ (401.5)
Net loss attributable to ATI after assumed conversions	$ (49.2)	$ (422.6)	$ (57.1)	$ (401.5)
Denominator for basic net loss per common share – weighted average shares	127.1	126.6	127.0	126.4
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	127.1	126.6	127.0	126.4
Basic net (loss) income attributable to ATI per common share (in dollars per share)	$ (0.39)	$ (3.34)	$ (0.45)	$ (3.18)
Diluted net (loss) income attributable to ATI per common share (in dollars per share)	$ (0.39)	$ (3.34)	$ (0.45)	$ (3.18)
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	25.7	20.5	25.7	20.4
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	$ 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%		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	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Convertible Senior Notes- Effect on denominator	0.0	0.0	0.0	0.0