CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 29, 2024	Oct. 01, 2023	Sep. 29, 2024	Oct. 01, 2023
Income Statement [Abstract]
Sales	$ 1,051.2	$ 1,025.6	$ 3,189.4	$ 3,109.7
Cost of sales	826.4	831.0	2,539.8	2,512.8
Gross profit	224.8	194.6	649.6	596.9
Selling and administrative expenses	82.4	69.8	253.3	235.8
Restructuring charges (credits)	0.5	(0.5)	(1.2)	2.2
Loss (gain) on asset sales and sales of businesses, net	(0.3)	0.1	(2.5)	0.8
Operating income	142.2	125.2	400.0	358.1
Nonoperating retirement benefit expense	(3.7)	(2.4)	(11.1)	(7.3)
Interest expense, net	(28.0)	(23.8)	(83.0)	(65.0)
Other income, net	4.4	0.0	5.2	1.3
Income before income taxes	114.9	99.0	311.1	287.1
Income tax provision	28.3	4.9	70.5	12.9
Net income	86.6	94.1	240.6	274.2
Less: Net income attributable to noncontrolling interests	3.9	3.9	9.9	9.1
Net income attributable to ATI	$ 82.7	$ 90.2	$ 230.7	$ 265.1
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.64	$ 0.70	$ 1.82	$ 2.06
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.57	$ 0.62	$ 1.61	$ 1.82