Per Share Information (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount	9.6	0
Earnings Per Share Reconciliation Abstract
Net income (loss) attributable to ATI	$ 10.0	$ 56.2
4.25% Convertible Notes due 2014	0	2.3
Net Income Loss Available To Common Stockholders Diluted, Total	$ 10.0	$ 58.5
Denominator for basic net income per common share - weighted average shares	106.6	105.9
Share-based compensation	0.5	0.9
4.25% Convertible Notes due 2014	0	9.6
Weighted Average Number Of Diluted Shares Outstanding, Total	107.1	116.4
Basic net income (loss) attributable to ATI per common share	$ 0.09	$ 0.53
Diluted net income (loss) attributable to ATI per common share	$ 0.09	$ 0.50