Consolidated Statements of Operations - USD ($) $ in Millions	12 Months Ended
	Dec. 28, 2025	Dec. 29, 2024	Dec. 31, 2023
Income Statement [Abstract]
Sales	$ 4,587.4	$ 4,362.1	$ 4,173.7
Cost of sales	3,580.4	3,463.9	3,371.1
Gross profit	1,007.0	898.2	802.6
Selling and administrative expenses	365.1	342.3	328.1
Restructuring (credits) charges	(1.9)	4.1	7.7
Loss (gain) on asset sales and sales of businesses, net	2.9	(57.1)	0.4
Operating income	640.9	608.9	466.4
Nonoperating retirement benefit expense	(34.6)	(29.0)	(79.7)
Interest expense, net	(98.6)	(108.2)	(92.8)
Other income, net	14.6	14.4	1.3
Income before income taxes	522.3	486.1	295.2
Income tax provision (benefit)	103.7	103.4	(128.2)
Net income	418.6	382.7	423.4
Less: Net income attributable to noncontrolling interests	14.3	14.9	12.6
Net income attributable to ATI	$ 404.3	$ 367.8	$ 410.8
Basic net income attributable to ATI per common share (in dollars per share)	$ 2.92	$ 2.82	$ 3.21
Diluted net income attributable to ATI per common share (in dollars per share)	$ 2.85	$ 2.55	$ 2.81