Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Earnings Per Share Reconciliation [Abstract]
Net income attributable to ATI	$ 75.1	$ 72.8	$ 90.1	$ 130.8
4.75% Convertible Senor Notes due 2022	3.2	3.2	6.5	6.4
Net income attributable to ATI after assumed conversions	$ 78.3	$ 76.0	$ 96.6	$ 137.2
Denominator for basic net income per common share – weighted average shares	125.9	125.2	125.7	125.1
Share-based compensation	0.6	0.7	0.6	0.6
Effective of dilutive securities: 4.75% Convertible Senior Notes due 2022	19.9	19.9	19.9	19.9
Denominator for diluted net income per common share – adjusted weighted average shares and assumed conversions	146.4	145.8	146.2	145.6
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.60	$ 0.58	$ 0.72	$ 1.05
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.54	$ 0.52	$ 0.66	$ 0.94
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share Amount	0.0	0.0	0.0	0.0
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Interest rate	4.75%		4.75%		4.75%