Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net (loss) income attributable to ATI	$ (422.6)	$ 75.1	$ (401.5)	$ 90.1
Net (loss) income attributable to ATI after assumed conversions	$ (422.6)	$ 78.3	$ (401.5)	$ 96.6
Denominator for basic net (loss) income per common share – weighted average shares	126.6	125.9	126.4	125.7
Share-based compensation	0.0	0.6	0.0	0.6
Denominator for diluted net (loss) income per common share – adjusted weighted average shares and assumed conversions	126.6	146.4	126.4	146.2
Basic net (loss) income attributable to ATI per common share (in dollars per share)	$ (3.34)	$ 0.60	$ (3.18)	$ 0.72
Diluted net (loss) income attributable to ATI per common share (in dollars per share)	$ (3.34)	$ 0.54	$ (3.18)	$ 0.66
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	20.5	0.0	20.4	0.0
Allegheny Technologies, Convertible Senior Notes, 4.75%, 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	$ 6.5
Convertible Senior Notes- Effect on denominator	0.0	19.9	0.0	19.9
Allegheny Technologies, Convertible Senior Notes, 3.5%, 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