CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Income Statement [Abstract]
Sales	$ 834.1	$ 692.5
Cost of sales	664.7	606.7
Gross profit	169.4	85.8
Selling and administrative expenses	75.2	54.0
Restructuring charges (credits)	(1.1)	0.0
Loss (gain) on asset sales and sales of businesses, net	18.3	0.0
Operating income	77.0	31.8
Nonoperating retirement benefit expense	(5.8)	(6.8)
Interest expense, net	(23.6)	(23.4)
Other income (expense), net	(7.5)	1.5
Income before income taxes	40.1	3.1
Income tax provision	4.9	5.5
Net income (loss)	35.2	(2.4)
Less: Net income attributable to noncontrolling interests	4.3	5.5
Net income (loss) attributable to ATI	$ 30.9	$ (7.9)
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.24	$ (0.06)
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.23	$ (0.06)