CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Sales	$ 880.2	$ 810.5	$ 1,746.1	$ 1,568.0
Cost of sales	767.9	762.3	1,521.0	1,553.0
Gross profit	112.3	48.2	225.1	15.0
Selling and administrative expenses	66.7	59.3	126.2	121.9
Restructuring charges	0.0	1.0	0.0	10.0
Operating income (loss)	45.6	(12.1)	98.9	(116.9)
Interest expense, net	(34.5)	(30.3)	(68.0)	(58.6)
Other income, net	0.2	1.0	3.5	1.8
Income (loss) before income taxes	11.3	(41.4)	34.4	(173.7)
Income tax benefit	(2.1)	(25.9)	(0.1)	(60.1)
Net income (loss)	13.4	(15.5)	34.5	(113.6)
Less: Net income attributable to noncontrolling interests	3.3	3.3	6.9	6.4
Net income (loss) attributable to ATI	$ 10.1	$ (18.8)	$ 27.6	$ (120.0)
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.09	$ (0.18)	$ 0.26	$ (1.12)
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	0.09	(0.18)	0.25	(1.12)
Dividends declared per common share (in dollars per share)	$ 0	$ 0.08	$ 0	$ 0.16