Per Share Information (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount	10,000,000	0	10,000,000	0
Earnings Per Share Reconciliation [Abstract]
Income (loss) from continuing operations attributable to ATI	$ (28.4)	$ 31.3	$ (15.0)	$ 134.5
4.25% Convertible Notes due 2014	0	2.1	0	6.5
Income (loss) from continuing operations available to ATI after assumed conversions	$ (28.4)	$ 33.4	$ (15.0)	$ 141.0
Denominator for basic net income per common share-weighted average shares	106,800,000	106,200,000	106,700,000	106,100,000
Share-based compensation	0	900,000	0	900,000
4.25% Convertible Notes due 2014	0	9,600,000	0	9,600,000
Denominator for diluted net income per common share – adjusted weighted average shares assuming conversions	106,800,000	116,700,000	106,700,000	116,600,000
Basic income (loss) from continuing operations attributable to ATI per common share (in dollars per share)	$ (0.27)	$ 0.30	$ (0.14)	$ 1.27
Diluted income (loss) from continuing operations attributable to ATI per common share (in dollars per share)	$ (0.27)	$ 0.29	$ (0.14)	$ 1.21