Consolidated Statements of Operations - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Jan. 01, 2023	Jan. 02, 2022
Income Statement [Abstract]
Sales	$ 4,173.7	$ 3,836.0	$ 2,799.8
Costs and expenses:
Cost of sales	3,371.1	3,121.8	2,466.6
Gross profit	802.6	714.2	333.2
Selling and administrative expenses	328.1	297.5	226.9
Restructuring charges (credits)	7.7	(4.8)	(11.3)
Loss on asset sales and sales of businesses, net	0.4	105.4	0.0
Operating income	466.4	316.1	117.6
Nonoperating retirement benefit income (expense)	(79.7)	138.4	260.0
Interest expense, net	(92.8)	(87.4)	(96.9)
Debt extinguishment charge	0.0	0.0	(65.5)
Other income (loss), net	1.3	(12.5)	18.2
Income before income taxes	295.2	354.6	233.4
Income tax provision (benefit)	(128.2)	15.5	26.8
Net income	423.4	339.1	206.6
Less: Net income attributable to noncontrolling interests	12.6	15.6	22.0
Net income attributable to ATI	$ 410.8	$ 323.5	$ 184.6
Basic net income (loss) per common share
Continuing operations attributable to ATI per common share (in dollars per share)	$ 3.21	$ 2.54	$ 1.45
Basic net income (loss) attributable to ATI per common share (in dollars per share)	3.21	2.54	1.45
Diluted net income (loss) per common share
Continuing operations attributable to ATI per common share (in dollars per share)	2.81	2.23	1.32
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ 2.81	$ 2.23	$ 1.32