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Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share Reconciliation [Abstract]
Net loss									$ (91.9)	$ (640.9)	$ (377.9)
Redeemable noncontrolling interest									0.0	0.0	(0.3)
Redeemable noncontrolling interest											0.0
Effect of dilutive securities: Convertible Senior Notes									0.0	0.0	0.0
Numerator for diluted net loss per common share - Loss attributable to ATI after assumed conversions									$ (91.9)	$ (640.9)	$ (378.2)
Denominator for basic net loss per common share - weighted average shares (shares)	118.6	108.9	108.9	108.8	108.9	108.9	108.9	109.0	110.1	107.3	107.3
Effect of dilutive securities: Share-based compensation									0.0	0.0	0.0
Effect of dilutive securities: Convertible Senior Notes									0.0	0.0	0.0
Denominator for diluted net loss per common share—adjusted weighted average shares and assumed conversions									110.1	107.3	107.3
Basic income (loss) attributable to ATI per common share (in dollars per share)									$ (0.83)	$ (5.97)	$ (3.53)
Diluted income (loss) attributable to ATI per common share (in dollars per share)									$ (0.83)	$ (5.97)	$ (3.53)
Anti-dilutive securities excluded from computation of earnings per share amount									20.8	13.1	0.8
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 [Member]
Earnings Per Share Reconciliation [Abstract]
Debt instrument interest rate stated percentage	4.75%				4.75%				4.75%	4.75%
Convertible Debt [Member] \| Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 [Member]
Earnings Per Share Reconciliation [Abstract]
Debt instrument interest rate stated percentage	4.75%								4.75%