Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Feb. 02, 2022	Dec. 31, 2021
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income (loss) attributable to ATI	$ 30.9	$ (7.9)
Net income (loss) attributable to ATI after assumed conversions	$ 34.9	$ (7.9)
Denominator for basic net income (loss) per common share – weighted average shares	126.4	126.8
Share-based compensation	1.8	0.0
Denominator for diluted net income (loss) per common share – adjusted weighted average shares and assumed conversions	152.8	126.8
Basic net (loss) income attributable to ATI per common share (in dollars per share)	$ 0.24	$ (0.06)
Diluted net (loss) income attributable to ATI per common share (in dollars per share)	$ 0.23	$ (0.06)
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	0.0	25.7
Purchase of treasury stock	$ (89.9)	$ 0.0
Stock repurchase plan, authorized amount			$ 150.0
Treasury Stock, Shares, Acquired	3.5
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	$ 0.0
Convertible Senior Notes- Effect on denominator	5.8	0.0
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	$ 2.9	$ 0.0
Convertible Senior Notes- Effect on denominator	18.8	0.0