Quarterly Financial Data (Unaudited) - 2023 and 2022 (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
Quarterly Financial Information Disclosure [Abstract]
Sales	$ 1,064.0	$ 1,025.6	$ 1,046.0	$ 1,038.1	$ 1,010.4	$ 1,032.0	$ 959.5	$ 834.1	$ 4,173.7	$ 3,836.0	$ 2,799.8
Operating income	108.3	125.2	120.3	112.6	108.7	113.2	17.2	77.0	466.4	316.1	117.6
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 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