CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Apr. 02, 2023
Income Statement [Abstract]
Sales	$ 1,042.9	$ 1,038.1
Cost of sales	845.5	844.9
Gross profit	197.4	193.2
Selling and administrative expenses	82.0	80.6
Restructuring charges	0.2	0.0
Operating income	115.2	112.6
Nonoperating retirement benefit expense	(3.7)	(2.4)
Interest expense, net	(26.6)	(19.9)
Other income, net	0.4	0.6
Income before income taxes	85.3	90.9
Income tax provision	16.9	4.3
Net income	68.4	86.6
Less: Net income attributable to noncontrolling interests	2.3	2.1
Net income attributable to ATI	$ 66.1	$ 84.5
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.52	$ 0.66
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.46	$ 0.58