CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
Sales	$ 770.5	$ 832.7	$ 2,338.5	$ 2,980.7
Cost of sales	720.3	861.4	2,273.3	2,822.9
Gross profit (loss)	50.2	(28.7)	65.2	157.8
Selling and administrative expenses	60.5	62.5	182.4	198.0
Restructuring charges	488.6	0.0	498.6	0.0
Operating loss	(498.9)	(91.2)	(615.8)	(40.2)
Interest expense, net	(32.6)	(27.5)	(91.2)	(81.0)
Other income, net	0.0	0.8	1.8	2.3
Loss before income taxes	(531.5)	(117.9)	(705.2)	(118.9)
Income tax provision (benefit)	(4.3)	23.4	(64.4)	23.7
Net income (loss)	(527.2)	(141.3)	(640.8)	(142.6)
Less: Net income attributable to noncontrolling interests	3.6	3.3	10.0	8.4
Net income (loss) attributable to ATI	$ (530.8)	$ (144.6)	$ (650.8)	$ (151.0)
Basic net loss attributable to ATI per common share (in dollars per share)	$ (4.95)	$ (1.35)	$ (6.07)	$ (1.41)
Diluted net loss attributable to ATI per common share (in dollars per share)	(4.95)	(1.35)	(6.07)	(1.41)
Dividends declared per common share (in dollars per share)	$ 0.08	$ 0.18	$ 0.24	$ 0.54