CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Income Statement [Abstract]
Sales	$ 1,025.6	$ 1,032.0	$ 3,109.7	$ 2,825.6
Cost of sales	831.0	848.2	2,512.8	2,297.1
Gross profit	194.6	183.8	596.9	528.5
Selling and administrative expenses	69.8	73.2	235.8	220.7
Restructuring charges (credits)	(0.5)	(2.6)	2.2	(5.0)
Loss on asset sales and sales of businesses, net	0.1	0.0	0.8	134.2
Operating income	125.2	113.2	358.1	178.6
Nonoperating retirement benefit expense	(16.9)	(6.5)	(50.6)	(18.9)
Interest expense, net	(23.8)	(20.8)	(65.0)	(67.8)
Other income (expense), net	0.0	(18.5)	1.3	(15.3)
Income before income taxes	84.5	67.4	243.8	76.6
Income tax provision	4.9	3.0	12.9	11.3
Net income	79.6	64.4	230.9	65.3
Less: Net income attributable to noncontrolling interests	3.9	3.3	9.1	11.3
Net income attributable to ATI	$ 75.7	$ 61.1	$ 221.8	$ 54.0
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.59	$ 0.47	$ 1.73	$ 0.43
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.52	$ 0.42	$ 1.53	$ 0.42