Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Antidilution
-URI http://asc.fasb.org/extlink&oid=6505113

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Diluted Earnings Per Share
-URI http://asc.fasb.org/extlink&oid=6510752

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Contingent Stock Agreement
-URI http://asc.fasb.org/extlink&oid=6508534

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Per Share Information (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share Reconciliation [Abstract]
Loss from continuing operations, net of tax	$ (226.9)	$ (144.6)	$ (16.4)	$ 10.0	$ 19.9	$ 0.0	$ (3.8)	$ (18.1)	$ (377.9)	$ (2.0)	$ (98.8)
Redeemable noncontrolling interest									(0.3)	(0.3)	0.0
Redeemable noncontrolling interest									0.0	0.0
Effect of dilutive securities: 4.25% Convertible Notes due 2014									0.0	0.0	0.0
Numerator for diluted net income (loss) per common share - Income (loss) from continuing operations attributable to ATI after assumed conversions									$ (378.2)	$ (2.3)	$ (98.8)
Denominator for basic net income per common share - weighted average shares (shares)	109,171,124	109,204,307	109,190,876	108,854,720	108,704,983	108,712,682	108,628,024	108,173,581	107,300,000	107,100,000	106,800,000
Effect of dilutive securities: Share-based compensation									0	0	0
Effect of dilutive securities: 4.25% Convertible Notes due 2014									0	0	0
Denominator for diluted net loss per common share—adjusted weighted average shares and assumed conversions									107,300,000	107,100,000	106,800,000
Basic income (loss) from continuing operations attributable to ATI per common share (in dollars per share)	$ (2.12)	$ (1.35)	$ (0.15)	$ 0.09	$ 0.18	$ 0.00	$ (0.03)	$ (0.17)	$ (3.53)	$ (0.02)	$ (0.93)
Diluted income (loss) from continuing operations attributable to ATI per common share (in dollars per share)	$ (2.12)	$ (1.35)	$ (0.15)	$ 0.09	$ 0.18	$ 0.00	$ (0.03)	$ (0.17)	$ (3.53)	$ (0.02)	$ (0.93)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount									800,000	4,700,000	10,000,000
Allegheny Technologies $402.5 million 4.25% Convertible Senior Notes due 2014 [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Debt instrument interest rate stated percentage											4.25%