CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Sales	$ 1,004.8	$ 979.0
Cost of sales	873.7	830.4
Gross profit	131.1	148.6
Selling and administrative expenses	68.0	67.1
Operating income	63.1	81.5
Nonoperating retirement benefit expense	(18.3)	(8.3)
Interest expense, net	(24.8)	(25.5)
Other (expense) income, net	(2.9)	17.8
Income before income taxes	17.1	65.5
Income tax provision	0.8	5.0
Net income (loss)	16.3	60.5
Less: Net income attributable to noncontrolling interests	1.3	2.5
Net income (loss) attributable to ATI	$ 15.0	$ 58.0
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.12	$ 0.46
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.12	$ 0.42