Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Sales	$ 909.9	$ 869.1	$ 880.2	$ 865.9	$ 796.1	$ 770.5	$ 810.5	$ 757.5	$ 3,525.1	$ 3,134.6	$ 3,719.6
Gross profit	130.6	93.3	112.3	112.8	97.3	50.2	48.2	(33.2)	449.0	162.5	60.3
Net income (loss)	5.2	(119.4)	13.4	21.1	13.7	(527.2)	(15.5)	(98.1)	(79.7)	(627.1)	(365.9)
Net income (loss) attributable to ATI	$ 1.7	$ (121.2)	$ 10.1	$ 17.5	$ 9.9	$ (530.8)	$ (18.8)	$ (101.2)	$ (91.9)	$ (640.9)	$ (377.9)
Basic income (loss) attributable to ATI per common share (in dollars per share)									$ (0.83)	$ (5.97)	$ (3.53)
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.01	$ (1.12)	$ 0.09	$ 0.16	$ 0.09	$ (4.95)	$ (0.18)	$ (0.94)	(0.83)	(5.97)	(3.53)
Diluted income (loss) attributable to ATI per common share (in dollars per share)									(0.83)	(5.97)	(3.53)
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.01	$ (1.12)	$ 0.09	$ 0.16	$ 0.09	$ (4.95)	$ (0.18)	$ (0.94)	$ (0.83)	$ (5.97)	$ (3.53)
Average shares outstanding (shares)	118.6	108.9	108.9	108.8	108.9	108.9	108.9	109.0	110.1	107.3	107.3