CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Sales	$ 869.1	$ 770.5	$ 2,615.2	$ 2,338.5
Cost of sales	775.8	720.3	2,296.8	2,273.3
Gross profit	93.3	50.2	318.4	65.2
Selling and administrative expenses	66.2	60.5	192.4	182.4
Impairment of goodwill	114.4	0.0	114.4	0.0
Restructuring charges	0.0	488.6	0.0	498.6
Operating income (loss)	(87.3)	(498.9)	11.6	(615.8)
Interest expense, net	(34.2)	(32.6)	(102.2)	(91.2)
Other income, net	0.2	0.0	3.7	1.8
Loss before income taxes	(121.3)	(531.5)	(86.9)	(705.2)
Income tax benefit	(1.9)	(4.3)	(2.0)	(64.4)
Net income (loss)	(119.4)	(527.2)	(84.9)	(640.8)
Less: Net income attributable to noncontrolling interests	1.8	3.6	8.7	10.0
Net income (loss) attributable to ATI	$ (121.2)	$ (530.8)	$ (93.6)	$ (650.8)
Basic net loss attributable to ATI per common share (in dollars per share)	$ (1.12)	$ (4.95)	$ (0.87)	$ (6.07)
Diluted net loss attributable to ATI per common share (in dollars per share)	(1.12)	(4.95)	(0.87)	(6.07)
Dividends declared per common share (in dollars per share)	$ 0	$ 0.08	$ 0	$ 0.24