Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income (loss) attributable to ATI	$ (50.1)	$ 111.0	$ (451.6)	$ 201.1
Net income (loss) attributable to ATI after assumed conversions	$ (50.1)	$ 114.2	$ (451.6)	$ 210.8
Denominator for basic net income (loss) per common share – weighted average shares	126.6	125.9	126.5	125.7
Share-based compensation	0.0	0.9	0.0	0.8
Denominator for diluted net income (loss) per common share – adjusted weighted average shares and assumed conversions	126.6	146.7	126.5	146.4
Basic net (loss) income attributable to ATI per common share (in dollars per share)	$ (0.40)	$ 0.88	$ (3.57)	$ 1.60
Diluted net (loss) income attributable to ATI per common share (in dollars per share)	$ (0.40)	$ 0.78	$ (3.57)	$ 1.44
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	25.0	0.0	21.9	0.0
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	$ 3.2	$ 0.0	$ 9.7
Convertible Senior Notes- Effect on denominator	0.0	19.9	0.0	19.9
Interest rate	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