Selected Quarterly Financial Data (Unaudited) (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
Quarterly Financial Information Disclosure [Abstract]
Sales	$ 1,047.5	$ 1,069.6	$ 1,119.0	$ 987.3	$ 915.3	$ 972.4	$ 1,056.8	$ 1,099.0	$ 4,223.4	$ 4,043.5	$ 4,666.9
Gross Profit	121.9	97.0	89.5	70.2	11.3	53.1	87.7	100.5
Income (loss) from continuing operations attributable to ATI	19.9	0	(3.8)	(18.1)	(83.8)	(28.4)	3.7	9.7	(2.0)	(98.8)	150.5
Net income (loss)	25.3	2.9	(0.7)	(17.9)	175.6	(32.2)	6.6	11.6	9.6	161.6	167.8
Net income (loss) attributable to ATI	$ 22.1	$ (0.7)	$ (4.0)	$ (20.0)	$ 173.4	$ (33.8)	$ 4.4	$ 10.0	$ (2.6)	$ 154.0	$ 158.4
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
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.20	$ (0.01)	$ (0.03)	$ (0.19)	$ 1.62	$ (0.32)	$ 0.04	$ 0.09	$ (0.03)	$ 1.44	$ 1.49
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
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.20	$ (0.01)	$ (0.03)	$ (0.19)	$ 1.62	$ (0.32)	$ 0.04	$ 0.09	$ (0.03)	$ 1.44	$ 1.43
Average shares outstanding (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