CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Sales	$ 725.7	$ 598.0	$ 2,034.4	$ 2,323.8
Cost of sales	643.2	559.9	1,823.4	2,076.2
Gross profit	82.5	38.1	211.0	247.6
Selling and administrative expenses	54.9	45.4	169.1	148.2
Impairment of goodwill	0.0	0.0	0.0	287.0
Restructuring charges (credits)	(2.3)	2.3	(8.5)	27.0
Operating income (loss)	29.9	(9.6)	50.4	(214.6)
Nonoperating retirement benefit income (expense)	57.9	(11.1)	44.3	(33.5)
Interest expense, net	(25.1)	(25.1)	(72.2)	(68.7)
Debt extinguishment charge	0.0	0.0	0.0	(21.5)
Other income (expense), net	14.5	(0.4)	17.4	(0.8)
Income (loss) before income taxes	77.2	(46.2)	39.9	(339.1)
Income tax provision	22.0	0.8	31.5	104.2
Net income (loss)	55.2	(47.0)	8.4	(443.3)
Less: Net income attributable to noncontrolling interests	6.5	3.1	16.8	8.3
Net income (loss) attributable to ATI	$ 48.7	$ (50.1)	$ (8.4)	$ (451.6)
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.38	$ (0.40)	$ (0.07)	$ (3.57)
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.35	$ (0.40)	$ (0.07)	$ (3.57)