CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended
	Mar. 30, 2025	Mar. 31, 2024
Income Statement [Abstract]
Sales	$ 1,144.4	$ 1,042.9
Cost of sales	908.6	845.5
Gross profit	235.8	197.4
Selling and administrative expenses	85.0	82.0
Restructuring charges	0.0	0.2
Loss on asset sales and sales of businesses, net	3.9	0.0
Operating income	146.9	115.2
Nonoperating retirement benefit expense	(3.9)	(3.7)
Interest expense, net	(23.0)	(26.6)
Other income, net	1.5	0.4
Income before income taxes	121.5	85.3
Income tax provision	21.0	16.9
Net income	100.5	68.4
Less: Net income attributable to noncontrolling interests	3.5	2.3
Net income attributable to ATI	$ 97.0	$ 66.1
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.68	$ 0.52
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.67	$ 0.46