Consolidated Statements of Operations - USD ($) $ in Millions	12 Months Ended
	Dec. 29, 2024	Dec. 31, 2023	Jan. 01, 2023
Income Statement [Abstract]
Sales	$ 4,362.1	$ 4,173.7	$ 3,836.0
Costs and expenses:
Cost of sales	3,463.9	3,371.1	3,121.8
Gross profit	898.2	802.6	714.2
Selling and administrative expenses	342.3	328.1	297.5
Restructuring charges (credits)	4.1	7.7	(4.8)
Loss (gain) on asset sales and sales of businesses, net	(57.1)	0.4	105.4
Operating income	608.9	466.4	316.1
Nonoperating retirement benefit income (expense)	(29.0)	(79.7)	138.4
Interest expense, net	(108.2)	(92.8)	(87.4)
Other income (loss), net	14.4	1.3	(12.5)
Income before income taxes	486.1	295.2	354.6
Income tax provision (benefit)	103.4	(128.2)	15.5
Net income	382.7	423.4	339.1
Less: Net income attributable to noncontrolling interests	14.9	12.6	15.6
Net income attributable to ATI	$ 367.8	$ 410.8	$ 323.5
Basic net income (loss) per common share
Continuing operations attributable to ATI per common share (in dollars per share)	$ 2.82	$ 3.21	$ 2.54
Basic net income (loss) attributable to ATI per common share (in dollars per share)	2.82	3.21	2.54
Diluted net income (loss) per common share
Continuing operations attributable to ATI per common share (in dollars per share)	2.55	2.81	2.23
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ 2.55	$ 2.81	$ 2.23