CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income Statement [Abstract]
Sales	$ 810.5	$ 1,022.5	$ 1,568.0	$ 2,148.0
Cost of sales	762.3	945.5	1,553.0	1,961.5
Gross profit	48.2	77.0	15.0	186.5
Selling and administrative expenses	59.3	72.4	121.9	135.5
Restructuring charges	1.0	0.0	10.0	0.0
Operating income (loss)	(12.1)	4.6	(116.9)	51.0
Interest expense, net	(30.3)	(26.8)	(58.6)	(53.5)
Other income, net	1.0	0.6	1.8	1.5
Loss before income taxes	(41.4)	(21.6)	(173.7)	(1.0)
Income tax provision (benefit)	(25.9)	(7.7)	(60.1)	0.3
Net income (loss)	(15.5)	(13.9)	(113.6)	(1.3)
Less: Net income attributable to noncontrolling interests	3.3	2.5	6.4	5.1
Net income (loss) attributable to ATI	$ (18.8)	$ (16.4)	$ (120.0)	$ (6.4)
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ (0.18)	$ (0.15)	$ (1.12)	$ (0.06)
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	(0.18)	(0.15)	(1.12)	(0.06)
Dividends declared per common share (in dollars per share)	$ 0.08	$ 0.18	$ 0.16	$ 0.36