Consolidated Statements of Operations - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Sales	$ 3,134.6	$ 3,719.6	$ 4,223.4
Costs and expenses:
Cost of sales	2,972.1	3,659.3	3,844.8
Gross profit	162.5	60.3	378.6
Selling and administrative expenses	247.7	238.8	272.5
Restructuring charges	527.2	64.3	0.0
Impairment of goodwill	0.0	126.6	0.0
Operating income (loss)	(612.4)	(369.4)	106.1
Interest expense, net	(124.0)	(110.2)	(108.7)
Other income, net	2.4	1.6	4.1
Income (loss) from continuing operations before income taxes	(734.0)	(478.0)	1.5
Income tax benefit	(106.9)	(112.1)	(8.7)
Income (loss) from continuing operations	(627.1)	(365.9)	10.2
Income (loss) from discontinued operations, net of tax	0.0	0.0	(0.6)
Net income (loss)	(627.1)	(365.9)	9.6
Less: Net income attributable to noncontrolling interests	13.8	12.0	12.2
Net loss attributable to ATI	$ (640.9)	$ (377.9)	$ (2.6)
Basic net income (loss) per common share
Continuing operations attributable to ATI per common share (in dollars per share)	$ (5.97)	$ (3.53)	$ (0.02)
Discontinued operations attributable to ATI per common share (in dollars per share)	0.00	0.00	(0.01)
Basic net income (loss) attributable to ATI per common share (in dollars per share)	(5.97)	(3.53)	(0.03)
Diluted net income (loss) per common share
Continuing operations attributable to ATI per common share (in dollars per share)	(5.97)	(3.53)	(0.02)
Discontinued operations attributable to ATI per common share (in dollars per share)	0.00	0.00	(0.01)
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ (5.97)	$ (3.53)	$ (0.03)
Loss from continuing operations, net of tax	$ (640.9)	$ (377.9)	$ (2.0)
Income (loss) from discontinued operations, net of tax	$ 0.0	$ 0.0	$ (0.6)