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Per Share Information (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share Reconciliation [Abstract]
Income (loss) from continuing operations, net of tax	$ 19.9	$ 0	$ (3.8)	$ (18.1)	$ (83.8)	$ (28.4)	$ 3.7	$ 9.7	$ (2.0)	$ (98.8)	$ 150.5
Redeemable noncontrolling interest									(0.3)us-gaap_TemporaryEquityOtherChanges	0us-gaap_TemporaryEquityOtherChanges	0us-gaap_TemporaryEquityOtherChanges
Redeemable noncontrolling interest									0
Effect of dilutive securities: 4.25% Convertible Notes due 2014									0	0	8.5
Numerator for diluted net income (loss) per common share - Income (loss) from continuing operations attributable to ATI after assumed conversions									$ (2.3)	$ (98.8)	$ 159.0
Denominator for basic net income per common share - weighted average shares (shares)	108,704,983	108,712,682	108,628,024	108,173,581	107,984,535	108,001,306	107,980,753	107,614,468	107,100,000	106,800,000	106,100,000
Effect of dilutive securities: Share-based compensation									0	0	900,000
Effect of dilutive securities: 4.25% Convertible Notes due 2014									0	0	9,600,000
Denominator for diluted net income per common share—adjusted weighted average shares and assumed conversions									107,100,000	106,800,000	116,600,000
Basic income (loss) from continuing operations attributable to ATI per common share (in dollars per share)	$ 0.18	$ 0.00	$ (0.03)	$ (0.17)	$ (0.79)	$ (0.27)	$ 0.04	$ 0.09	$ (0.02)	$ (0.93)	$ 1.42
Diluted income (loss) from continuing operations attributable to ATI per common share (in dollars per share)	$ 0.18	$ 0.00	$ (0.03)	$ (0.17)	$ (0.79)	$ (0.27)	$ 0.04	$ 0.09	$ (0.02)	$ (0.93)	$ 1.36
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of earnings per share amount									4,700,000	10,000,000	0
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%					4.25%