Summary of Significant Accounting Policies - Change in Accounting Principle (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 29, 2024	Dec. 31, 2023	Jan. 01, 2023	Jan. 02, 2022
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loss (gain) on asset sales and sales of businesses, net	$ (57.1)	$ 0.4	$ 105.4
Operating Income (Loss)	608.9	466.4	316.1
Nonoperating retirement benefit income (expense)	(29.0)	(79.7)	138.4
Income tax provision (benefit)	103.4	(128.2)	15.5
Net income	382.7	423.4	339.1
Net loss (gain) from sales of businesses	(52.9)	0.6	112.2
Net income attributable to ATI	$ 367.8	$ 410.8	$ 323.5
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ 2.82	$ 3.21	$ 2.54
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ 2.55	$ 2.81	$ 2.23
Amortization of net actuarial loss	$ 5.2	$ 6.0	$ 13.2
Settlement loss (gain) included in net income (loss)	0.0	1.1	0.7
Income taxes on postretirement benefit plans	0.5	0.3	0.0
Total	2.0	2.4	68.1
Other Comprehensive Income (Loss), Net of Tax	(7.9)	(16.2)	55.6
Retained earnings (loss)	64.3	(70.1)
Accumulated other comprehensive loss, net of tax	(89.5)	(83.2)	(67.4)	$ (141.3)
Deferred taxes	86.4	(138.2)	(0.1)
Retirement benefits	4.4	53.8	(159.2)
Stockholders' equity, including portion attributable to noncontrolling interest	1,955.2	1,480.5	1,157.2	832.7
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income	367.8	410.8	323.5
Stockholders' equity, including portion attributable to noncontrolling interest	64.3	(70.1)	(480.9)	(777.7)
Accumulated Other Comprehensive Income (Loss)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Comprehensive Income (Loss), Net of Tax	(6.3)	(15.8)	73.9
Stockholders' equity, including portion attributable to noncontrolling interest	$ (89.5)	$ (83.2)	$ (67.4)	$ (141.3)