CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Sales	$ 598.0	$ 1,018.7	$ 2,323.8	$ 3,103.9
Cost of sales	559.9	859.0	2,076.2	2,635.4
Gross profit	38.1	159.7	247.6	468.5
Selling and administrative expenses	45.4	65.2	148.2	200.9
Impairment of goodwill	0.0	0.0	287.0	0.0
Restructuring charges	2.3	0.0	27.0	0.0
Operating income (loss)	(9.6)	94.5	(214.6)	267.6
Nonoperating retirement benefit expense	(11.1)	(18.5)	(33.5)	(55.2)
Interest expense, net	(25.1)	(24.2)	(68.7)	(74.9)
Debt extinguishment charge	0.0	0.0	(21.5)	0.0
Other income (expense), net	(0.4)	67.2	(0.8)	82.9
Income (loss) before income taxes	(46.2)	119.0	(339.1)	220.4
Income tax provision	0.8	3.7	104.2	10.3
Net income (loss)	(47.0)	115.3	(443.3)	210.1
Less: Net income attributable to noncontrolling interests	3.1	4.3	8.3	9.0
Net income (loss) attributable to ATI	$ (50.1)	$ 111.0	$ (451.6)	$ 201.1
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ (0.40)	$ 0.88	$ (3.57)	$ 1.60
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ (0.40)	$ 0.78	$ (3.57)	$ 1.44