Summary of Significant Accounting Policies - Change in Accounting Principle (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2023	Oct. 01, 2023	Jul. 02, 2023	Apr. 02, 2023	Jan. 01, 2023	Oct. 02, 2022	Jul. 03, 2022	Apr. 03, 2022	Dec. 31, 2023	Jan. 01, 2023	Jan. 02, 2022	Jan. 04, 2021	Jan. 03, 2021
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loss on asset sales and sales of businesses, net									$ 0.4	$ 105.4	$ 0.0
Operating income	$ 108.3	$ 125.2	$ 120.3	$ 112.6	$ 108.7	$ 113.2	$ 17.2	$ 77.0	466.4	316.1	117.6
Nonoperating retirement benefit income (expense)									(79.7)	138.4	260.0
Income (loss) before income taxes									295.2	354.6	233.4
Income tax provision (benefit)									(128.2)	15.5	26.8
Net income	149.2	94.1	93.5	86.6	197.3	80.2	10.4	51.2	423.4	339.1	206.6
Net loss (gain) from sales of businesses									0.6	112.2	(13.8)
Net income attributable to ATI	$ 145.7	$ 90.2	$ 90.4	$ 84.5	$ 193.0	$ 76.9	$ 6.7	$ 46.9	$ 410.8	$ 323.5	$ 184.6
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ 1.15	$ 0.70	$ 0.70	$ 0.66	$ 1.49	$ 0.59	$ 0.05	$ 0.37	$ 3.21	$ 2.54	$ 1.45
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.99	$ 0.62	$ 0.62	$ 0.58	$ 1.30	$ 0.53	$ 0.05	$ 0.33	$ 2.81	$ 2.23	$ 1.32
Amortization of net actuarial loss									$ 6.0	$ 13.2	$ 13.9
Net loss arising during the period									(3.8)	54.7	8.7
Settlement loss (gain) included in net income (loss)									1.1	0.7	(21.9)
Income taxes on postretirement benefit plans									0.3	0.0	(15.5)
Total									2.4	68.1	14.4
Other comprehensive income (loss), net of tax									(16.2)	55.6	13.9
Retained loss	$ (70.1)				$ (480.9)				(70.1)	(480.9)	(777.7)
Accumulated other comprehensive loss, net of tax	(83.2)				(67.4)				(83.2)	(67.4)	(141.3)		$ (1,223.6)
Deferred taxes									(138.2)	(0.1)	1.0
Retirement benefits									53.8	(159.2)	(261.9)
Stockholders' equity, including portion attributable to noncontrolling interest	1,480.5				1,157.2				1,480.5	1,157.2	832.7		641.4
Change in Accounting Principle, Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest													0.0
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income									410.8	323.5	184.6
Stockholders' equity, including portion attributable to noncontrolling interest	(70.1)				(480.9)				(70.1)	(480.9)	(777.7)		106.5
Retained Earnings | Change in Accounting Principle, Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest												$ (1,073.2)	(1,073.2)
Retained Earnings | Accounting Standards Update 2020-06
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest													4.4
Accumulated Other Comprehensive Income (Loss)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other comprehensive income (loss), net of tax									(15.8)	73.9	9.1
Stockholders' equity, including portion attributable to noncontrolling interest	(83.2)				(67.4)				(83.2)	(67.4)	(141.3)		(1,223.6)
Accumulated Other Comprehensive Income (Loss) | Change in Accounting Principle, Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest												1,073.2	$ 1,073.2
Reported Under The Previous Policy
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loss on asset sales and sales of businesses, net										134.2
Operating income										287.3
Nonoperating retirement benefit income (expense)									(1,036.6)	(25.4)
Income (loss) before income taxes									(661.7)	162.0
Income tax provision (benefit)									(342.5)
Net income									(319.2)	146.5
Net loss (gain) from sales of businesses										141.0
Net income attributable to ATI									$ (331.8)	$ 130.9
Basic net income (loss) attributable to ATI per common share (in dollars per share)									$ (2.59)	$ 1.03
Diluted net income (loss) attributable to ATI per common share (in dollars per share)									$ (2.59)	$ 0.96
Amortization of net actuarial loss									$ 55.7	$ 76.7
Net loss arising during the period									(71.4)	155.0
Settlement loss (gain) included in net income (loss)									975.9	29.5
Income taxes on postretirement benefit plans									214.6
Total									745.0	260.7
Other comprehensive income (loss), net of tax									726.4	248.2
Retained loss	(154.9)				176.9				(154.9)	176.9
Accumulated other comprehensive loss, net of tax	1.6				(725.2)				1.6	(725.2)
Deferred taxes									(352.5)
Retirement benefits									1,010.7	4.6
Reported Under The Previous Policy | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income									(331.8)	130.9
Stockholders' equity, including portion attributable to noncontrolling interest	(154.9)				176.9				(154.9)	176.9
Reported Under The Previous Policy | Accumulated Other Comprehensive Income (Loss)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other comprehensive income (loss), net of tax									726.8	266.5
Stockholders' equity, including portion attributable to noncontrolling interest	1.6				(725.2)				1.6	(725.2)
Revision of Prior Period, Change in Accounting Principle, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loss on asset sales and sales of businesses, net										(28.8)
Operating income										28.8
Nonoperating retirement benefit income (expense)									956.9	163.8
Income (loss) before income taxes									956.9	192.6
Income tax provision (benefit)									214.3
Net income									742.6	192.6
Net loss (gain) from sales of businesses										(28.8)
Net income attributable to ATI									$ 742.6	$ 192.6
Basic net income (loss) attributable to ATI per common share (in dollars per share)									$ 5.80	$ 1.51
Diluted net income (loss) attributable to ATI per common share (in dollars per share)									$ 5.40	$ 1.27
Amortization of net actuarial loss									$ (49.7)	$ (63.5)
Net loss arising during the period									67.6	(100.3)
Settlement loss (gain) included in net income (loss)									(974.8)	(28.8)
Income taxes on postretirement benefit plans									(214.3)
Total									(742.6)	(192.6)
Other comprehensive income (loss), net of tax									(742.6)	(192.6)
Retained loss	84.8				(657.8)				84.8	(657.8)
Accumulated other comprehensive loss, net of tax	(84.8)				657.8				(84.8)	657.8
Deferred taxes									214.3
Retirement benefits									(956.9)	(163.8)
Revision of Prior Period, Change in Accounting Principle, Adjustment | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income									742.6	192.6
Stockholders' equity, including portion attributable to noncontrolling interest	84.8				(657.8)				84.8	(657.8)
Revision of Prior Period, Change in Accounting Principle, Adjustment | Accumulated Other Comprehensive Income (Loss)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other comprehensive income (loss), net of tax									(742.6)	(192.6)
Stockholders' equity, including portion attributable to noncontrolling interest	(84.8)				657.8				$ (84.8)	$ 657.8
As Computed Under Previous Policy
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating income							$ (11.6)
Nonoperating retirement benefit income (expense)											37.2
Income (loss) before income taxes											10.6
Net income	(550.1)	$ 79.6	$ 79.1	$ 72.2	81.2	$ 64.4	(34.3)	$ 35.2			(16.2)
Net income attributable to ATI	$ (553.6)	$ 75.7	$ 76.0	$ 70.1	$ 76.9	$ 61.1	$ (38.0)	$ 30.9			$ (38.2)
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ (4.35)	$ 0.59	$ 0.59	$ 0.55	$ 0.60	$ 0.47	$ (0.31)	$ 0.24			$ (0.30)
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ (4.35)	$ 0.52	$ 0.52	$ 0.48	$ 0.53	$ 0.42	$ (0.31)	$ 0.23			$ (0.30)
Amortization of net actuarial loss											$ 89.5
Net loss arising during the period											155.9
Total											237.2
Other comprehensive income (loss), net of tax											236.7
Retained loss											72.7
Accumulated other comprehensive loss, net of tax											(991.7)
Retirement benefits											(39.1)
As Computed Under Previous Policy | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income											(38.2)
Stockholders' equity, including portion attributable to noncontrolling interest											72.7
As Computed Under Previous Policy | Retained Earnings | Change in Accounting Principle, Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest												0.0
As Computed Under Previous Policy | Accumulated Other Comprehensive Income (Loss)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other comprehensive income (loss), net of tax											231.9
Stockholders' equity, including portion attributable to noncontrolling interest											(991.7)
As Computed Under Previous Policy | Accumulated Other Comprehensive Income (Loss) | Change in Accounting Principle, Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest												0.0
Effect of Accounting Change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating income							$ 28.8
Nonoperating retirement benefit income (expense)											222.8
Income (loss) before income taxes											222.8
Net income	$ 699.3	$ 14.5	$ 14.4	$ 14.4	$ 116.1	$ 15.8	44.7	$ 16.0			222.8
Net income attributable to ATI	$ 699.3	$ 14.5	$ 14.4	$ 14.4	$ 116.1	$ 15.8	$ 44.7	$ 16.0			$ 222.8
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ 5.50	$ 0.11	$ 0.11	$ 0.11	$ 0.89	$ 0.12	$ 0.36	$ 0.13			$ 1.75
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ 5.34	$ 0.10	$ 0.10	$ 0.10	$ 0.77	$ 0.11	$ 0.36	$ 0.10			$ 1.62
Amortization of net actuarial loss											$ (75.6)
Net loss arising during the period											(147.2)
Total											(222.8)
Other comprehensive income (loss), net of tax											(222.8)
Retained loss											(850.4)	1,070.0
Accumulated other comprehensive loss, net of tax											850.4
Retirement benefits											(222.8)
Effect of Accounting Change | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income											222.8
Stockholders' equity, including portion attributable to noncontrolling interest											(850.4)
Effect of Accounting Change | Retained Earnings | Change in Accounting Principle, Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest												(1,073.2)
Effect of Accounting Change | Accumulated Other Comprehensive Income (Loss)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other comprehensive income (loss), net of tax											(222.8)
Stockholders' equity, including portion attributable to noncontrolling interest											$ 850.4
Effect of Accounting Change | Accumulated Other Comprehensive Income (Loss) | Change in Accounting Principle, Other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity, including portion attributable to noncontrolling interest												$ 1,073.2