CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Income Statement [Abstract]
Sales	$ 1,032.0	$ 725.7	$ 2,825.6	$ 2,034.4
Cost of sales	848.2	643.2	2,297.1	1,823.4
Gross profit	183.8	82.5	528.5	211.0
Selling and administrative expenses	73.2	54.9	220.7	169.1
Restructuring credits	(2.6)	(2.3)	(5.0)	(8.5)
Loss on asset sales and sales of businesses, net	0.0	0.0	134.2	0.0
Operating income	113.2	29.9	178.6	50.4
Nonoperating retirement benefit income (expense)	(6.5)	57.9	(18.9)	44.3
Interest expense, net	(20.8)	(25.1)	(67.8)	(72.2)
Other income (expense), net	(18.5)	14.5	(15.3)	17.4
Income before income taxes	67.4	77.2	76.6	39.9
Income tax provision	3.0	22.0	11.3	31.5
Net income	64.4	55.2	65.3	8.4
Less: Net income attributable to noncontrolling interests	3.3	6.5	11.3	16.8
Net income (loss) attributable to ATI	$ 61.1	$ 48.7	$ 54.0	$ (8.4)
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.47	$ 0.38	$ 0.43	$ (0.07)
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.42	$ 0.35	$ 0.42	$ (0.07)