CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 28, 2025	Sep. 29, 2024	Sep. 28, 2025	Sep. 29, 2024
Income Statement [Abstract]
Sales	$ 1,125.5	$ 1,051.2	$ 3,410.3	$ 3,189.4
Cost of sales	870.2	826.4	2,676.7	2,539.8
Gross profit	255.3	224.8	733.6	649.6
Selling and administrative expenses	94.6	82.4	262.4	253.3
Restructuring (credits) charges	(0.4)	0.5	(1.7)	(1.2)
(Gain) loss on asset sales and sales of businesses, net	(1.3)	(0.3)	2.6	(2.5)
Operating income	162.4	142.2	470.3	400.0
Nonoperating retirement benefit expense	(3.9)	(3.7)	(11.9)	(11.1)
Interest expense, net	(26.1)	(28.0)	(74.5)	(83.0)
Other income, net	12.2	4.4	15.5	5.2
Income before income taxes	144.6	114.9	399.4	311.1
Income tax provision	31.0	28.3	81.3	70.5
Net income	113.6	86.6	318.1	240.6
Less: Net income attributable to noncontrolling interests	3.6	3.9	10.4	9.9
Net income attributable to ATI	$ 110.0	$ 82.7	$ 307.7	$ 230.7
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.80	$ 0.64	$ 2.21	$ 1.82
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.78	$ 0.57	$ 2.16	$ 1.61