CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Sales	$ 979.0	$ 865.9
Cost of sales	830.4	741.1
Gross profit	148.6	124.8
Selling and administrative expenses	67.1	57.9
Operating income	81.5	66.9
Nonoperating retirement benefit expense	(8.3)	(13.6)
Interest expense, net	(25.5)	(33.5)
Other income, net	17.8	3.3
Income before income taxes	65.5	23.1
Income tax provision	5.0	2.0
Net income (loss)	60.5	21.1
Less: Net income attributable to noncontrolling interests	2.5	3.6
Net income (loss) attributable to ATI	$ 58.0	$ 17.5
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.46	$ 0.16
Diluted net income attributable to ATI per common share (in dollars per share)	0.42	0.16
Dividends declared per common share (in dollars per share)	$ 0	$ 0