CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Income Statement [Abstract]
Sales	$ 959.5	$ 616.2	$ 1,793.6	$ 1,308.7
Cost of sales	784.2	573.5	1,448.9	1,180.2
Gross profit	175.3	42.7	344.7	128.5
Selling and administrative expenses	72.3	60.2	147.5	114.2
Restructuring credits	(1.3)	(6.2)	(2.4)	(6.2)
Loss on asset sales and sales of businesses, net	115.9	0.0	134.2	0.0
Operating income (loss)	(11.6)	(11.3)	65.4	20.5
Nonoperating retirement benefit expense	(6.6)	(6.8)	(12.4)	(13.6)
Interest expense, net	(23.4)	(23.7)	(47.0)	(47.1)
Other income, net	10.7	1.4	3.2	2.9
Income (loss) before income taxes	(30.9)	(40.4)	9.2	(37.3)
Income tax provision	3.4	4.0	8.3	9.5
Net income (loss)	(34.3)	(44.4)	0.9	(46.8)
Less: Net income attributable to noncontrolling interests	3.7	4.8	8.0	10.3
Net loss attributable to ATI	$ (38.0)	$ (49.2)	$ (7.1)	$ (57.1)
Basic net loss attributable to ATI per common share (in dollars per share)	$ (0.31)	$ (0.39)	$ (0.06)	$ (0.45)
Diluted net loss attributable to ATI per common share (in dollars per share)	$ (0.31)	$ (0.39)	$ (0.06)	$ (0.45)