Selected Quarterly Financial Data (Unaudited) (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
Quarterly Financial Information Disclosure [Abstract]
Sales	$ 738.9	$ 832.7	$ 1,022.5	$ 1,125.5	$ 1,047.5	$ 1,069.6	$ 1,119.0	$ 987.3	$ 3,719.6	$ 4,223.4	$ 4,043.5
Gross Profit	(97.5)	(28.7)	77.0	109.5	121.9	97.0	89.5	70.2
Income (loss) from continuing operations attributable to ATI	(226.9)	(144.6)	(16.4)	10.0	19.9	0.0	(3.8)	(18.1)	(377.9)	(2.0)	(98.8)
Net income (loss)	(223.3)	(141.3)	(13.9)	12.6	25.3	2.9	(0.7)	(17.9)	(365.9)	9.6	161.6
Net income (loss) attributable to ATI	$ (226.9)	$ (144.6)	$ (16.4)	$ 10.0	$ 22.1	$ (0.7)	$ (4.0)	$ (20.0)	$ (377.9)	$ (2.6)	$ 154.0
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)
Basic net income attributable to ATI per common share (in dollars per share)	(2.12)	(1.35)	(0.15)	0.09	0.20	(0.01)	(0.03)	(0.19)	(3.53)	(0.03)	1.44
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)
Diluted net income attributable to ATI per common share (in dollars per share)	$ (2.12)	$ (1.35)	$ (0.15)	$ 0.09	$ 0.20	$ (0.01)	$ (0.03)	$ (0.19)	$ (3.53)	$ (0.03)	$ 1.44
Average shares outstanding (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