CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jul. 02, 2023	Jun. 30, 2024	Jul. 02, 2023
Income Statement [Abstract]
Sales	$ 1,095.3	$ 1,046.0	$ 2,138.2	$ 2,084.1
Cost of sales	867.9	836.9	1,713.4	1,681.8
Gross profit	227.4	209.1	424.8	402.3
Selling and administrative expenses	88.9	85.4	170.9	166.0
Restructuring charges (credits)	(1.9)	2.7	(1.7)	2.7
Loss (gain) on asset sales and sales of businesses, net	(2.2)	0.7	(2.2)	0.7
Operating income	142.6	120.3	257.8	232.9
Nonoperating retirement benefit expense	(3.7)	(2.5)	(7.4)	(4.9)
Interest expense, net	(28.4)	(21.3)	(55.0)	(41.2)
Other income, net	0.4	0.7	0.8	1.3
Income before income taxes	110.9	97.2	196.2	188.1
Income tax provision	25.3	3.7	42.2	8.0
Net income	85.6	93.5	154.0	180.1
Less: Net income attributable to noncontrolling interests	3.7	3.1	6.0	5.2
Net income attributable to ATI	$ 81.9	$ 90.4	$ 148.0	$ 174.9
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.66	$ 0.70	$ 1.18	$ 1.36
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.58	$ 0.62	$ 1.04	$ 1.20