CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended
	Mar. 29, 2026	Mar. 30, 2025
Income Statement [Abstract]
Sales	$ 1,151.5	$ 1,144.4
Cost of sales	888.6	908.6
Gross profit	262.9	235.8
Selling and administrative expenses	92.1	85.0
Restructuring charges	7.0	0.0
Loss on asset sales and sales of businesses, net	0.0	3.9
Operating income	163.8	146.9
Nonoperating retirement benefit expense	(4.3)	(3.9)
Interest expense, net	(23.7)	(23.0)
Other income, net	0.8	1.5
Income before income taxes	136.6	121.5
Income tax provision	16.1	21.0
Net income	120.5	100.5
Less: Net income attributable to noncontrolling interests	2.3	3.5
Net income attributable to ATI	$ 118.2	$ 97.0
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.86	$ 0.68
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.85	$ 0.67