Per Share Information (Details) - USD ($) $ / shares in Units, shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share Reconciliation [Abstract]
Loss attributable to ATI	$ (530,800,000)	$ (144,600,000)	$ (650,800,000)	$ (151,000,000)
Redeemable noncontrolling interest	0	(200,000)	0	(300,000)
4.75% Convertible Senor Notes due 2022	0	0	0	0
Loss attributable to ATI after assumed conversions	$ (530,800,000)	$ (144,800,000)	$ (650,800,000)	$ (151,300,000)
Denominator for basic net loss per common share – weighted average shares	107.3	107.3	107.3	107.3
Share-based compensation	0.0	0.0	0.0	0.0
Effective of dilutive securities: 4.75% Convertible Senior Notes due 2022	0.0	0.0	0.0	0.0
Denominator for diluted net loss per common share – adjusted weighted average shares and assumed conversions	107.3	107.3	107.3	107.3
Basic loss attributable to ATI per common share (in dollars per share)	$ (4.95)	$ (1.35)	$ (6.07)	$ (1.41)
Diluted loss attributable to ATI per common share (in dollars per share)	$ (4.95)	$ (1.35)	$ (6.07)	$ (1.41)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount	21.0	0.8	10.3	0.9