Statements of Changes in Consolidated Equity - USD ($) $ in Millions	Total	ATI Inc., Convertible Senior Notes, 3.5%, Due 2025 Convertible Debt [Member]	Common Stock	Additional Paid-In Capital	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interests
Beginning balance at Jan. 03, 2021	$ 641.4		$ 12.7	$ 1,625.5	$ 106.5	$ 0.0	$ (1,223.6)	$ 120.3
Beginning balance (Change in Accounting Principle, Other) at Jan. 03, 2021	0.0				(1,073.2)		1,073.2
Beginning balance (Accounting Standards Update 2020-06) at Jan. 03, 2021		$ (45.4)		(49.8)	4.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	206.6				184.6			22.0
Other comprehensive income (loss), net of tax	$ 13.9						9.1	4.8
Accounting Standards Update [Extensible Enumeration]	Accounting Standards Update 2020-06
Employee Stock Plans	$ 16.2			21.0		(4.8)
Ending balance at Jan. 02, 2022	832.7		12.7	1,596.7	(777.7)	(4.8)	(141.3)	147.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	339.1				323.5			15.6
Other comprehensive income (loss), net of tax	55.6						73.9	(18.3)
Purchase of treasury stock	(139.9)					(139.9)
Conversion of convertible notes	82.5		0.3	45.4	(26.7)	63.5
Dividends paid to noncontrolling interest	(34.0)							(34.0)
Sales of subsidiary shares to noncontrolling interest	0.9							0.9
Employee Stock Plans	20.3		0.1	26.0	0.0	(5.8)
Ending balance at Jan. 01, 2023	1,157.2		13.1	1,668.1	(480.9)	(87.0)	(67.4)	111.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	423.4				410.8			12.6
Other comprehensive income (loss), net of tax	(16.2)						(15.8)	(0.4)
Purchase of treasury stock	(85.8)					(85.8)
Dividends paid to noncontrolling interest	(16.0)							(16.0)
Employee Stock Plans	17.9		0.1	29.0		(11.2)
Ending balance at Dec. 31, 2023	$ 1,480.5		$ 13.2	$ 1,697.1	$ (70.1)	$ (184.0)	$ (83.2)	$ 107.5