CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Sales	$ 955.5	$ 1,004.8
Cost of sales	820.7	873.7
Gross profit	134.8	131.1
Selling and administrative expenses	58.4	68.0
Restructuring charges	8.0	0.0
Operating income	68.4	63.1
Nonoperating retirement benefit expense	(11.2)	(18.3)
Interest expense, net	(21.9)	(24.8)
Other expense, net	(0.9)	(2.9)
Income before income taxes	34.4	17.1
Income tax provision	10.8	0.8
Net income	23.6	16.3
Less: Net income attributable to noncontrolling interests	2.5	1.3
Net income attributable to ATI	$ 21.1	$ 15.0
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.17	$ 0.12
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.16	$ 0.12