CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
Sales	$ 692.5	$ 955.5
Cost of sales	606.7	820.7
Gross profit	85.8	134.8
Selling and administrative expenses	54.0	58.4
Restructuring charges	0.0	8.0
Operating income	31.8	68.4
Nonoperating retirement benefit expense	(6.8)	(11.2)
Interest expense, net	(23.4)	(21.9)
Other income (expense), net	1.5	(0.9)
Income before income taxes	3.1	34.4
Income tax provision	5.5	10.8
Net income (loss)	(2.4)	23.6
Less: Net income attributable to noncontrolling interests	5.5	2.5
Net income (loss) attributable to ATI	$ (7.9)	$ 21.1
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ (0.06)	$ 0.17
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ (0.06)	$ 0.16