CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Income Statement [Abstract]
Sales	$ 1,046.0	$ 959.5	$ 2,084.1	$ 1,793.6
Cost of sales	836.9	784.2	1,681.8	1,448.9
Gross profit	209.1	175.3	402.3	344.7
Selling and administrative expenses	85.4	72.3	166.0	147.5
Restructuring charges (credits)	2.7	(1.3)	2.7	(2.4)
Loss on asset sales and sales of businesses, net	0.7	115.9	0.7	134.2
Operating income (loss)	120.3	(11.6)	232.9	65.4
Nonoperating retirement benefit expense	(16.9)	(6.6)	(33.7)	(12.4)
Interest expense, net	(21.3)	(23.4)	(41.2)	(47.0)
Other income, net	0.7	10.7	1.3	3.2
Income (loss) before income taxes	82.8	(30.9)	159.3	9.2
Income tax provision	3.7	3.4	8.0	8.3
Net income (loss)	79.1	(34.3)	151.3	0.9
Less: Net income attributable to noncontrolling interests	3.1	3.7	5.2	8.0
Net income (loss) attributable to ATI	$ 76.0	$ (38.0)	$ 146.1	$ (7.1)
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.59	$ (0.31)	$ 1.14	$ (0.06)
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.52	$ (0.31)	$ 1.01	$ (0.06)