CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Income Statement [Abstract]
Sales	$ 1,038.1	$ 834.1
Cost of sales	844.9	664.7
Gross profit	193.2	169.4
Selling and administrative expenses	80.6	75.2
Restructuring credits	0.0	(1.1)
Loss on asset sales and sales of businesses, net	0.0	18.3
Operating income	112.6	77.0
Nonoperating retirement benefit expense	(16.8)	(5.8)
Interest expense, net	(19.9)	(23.6)
Other income (expense), net	0.6	(7.5)
Income before income taxes	76.5	40.1
Income tax provision	4.3	4.9
Net income	72.2	35.2
Less: Net income attributable to noncontrolling interests	2.1	4.3
Net income attributable to ATI	$ 70.1	$ 30.9
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.55	$ 0.24
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.48	$ 0.23