CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 29, 2025	Jun. 30, 2024	Jun. 29, 2025	Jun. 30, 2024
Income Statement [Abstract]
Sales	$ 1,140.4	$ 1,095.3	$ 2,284.8	$ 2,138.2
Cost of sales	897.9	867.9	1,806.5	1,713.4
Gross profit	242.5	227.4	478.3	424.8
Selling and administrative expenses	82.8	88.9	167.8	170.9
Restructuring credits	(1.3)	(1.9)	(1.3)	(1.7)
Loss (gain) on asset sales and sales of businesses, net	0.0	(2.2)	3.9	(2.2)
Operating income	161.0	142.6	307.9	257.8
Nonoperating retirement benefit expense	(4.1)	(3.7)	(8.0)	(7.4)
Interest expense, net	(25.4)	(28.4)	(48.4)	(55.0)
Other income, net	1.8	0.4	3.3	0.8
Income before income taxes	133.3	110.9	254.8	196.2
Income tax provision	29.3	25.3	50.3	42.2
Net income	104.0	85.6	204.5	154.0
Less: Net income attributable to noncontrolling interests	3.3	3.7	6.8	6.0
Net income attributable to ATI	$ 100.7	$ 81.9	$ 197.7	$ 148.0
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.72	$ 0.66	$ 1.40	$ 1.18
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.70	$ 0.58	$ 1.38	$ 1.04