CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Sales	$ 770.3	$ 1,080.4	$ 1,725.8	$ 2,085.2
Cost of sales	695.6	902.7	1,516.3	1,776.4
Gross profit	74.7	177.7	209.5	308.8
Selling and administrative expenses	44.4	67.7	102.8	135.7
Impairment of goodwill	287.0	0.0	287.0	0.0
Restructuring charges	16.7	0.0	24.7	0.0
Operating (loss) income	(273.4)	110.0	(205.0)	173.1
Nonoperating retirement benefit expense	(11.2)	(18.4)	(22.4)	(36.7)
Interest expense, net	(21.7)	(25.9)	(43.6)	(50.7)
Debt extinguishment charge	(21.5)	0.0	(21.5)	0.0
Other income (expense), net	0.5	18.6	(0.4)	15.7
(Loss) income before income taxes	(327.3)	84.3	(292.9)	101.4
Income tax provision	92.6	5.8	103.4	6.6
Net (loss) income	(419.9)	78.5	(396.3)	94.8
Less: Net income attributable to noncontrolling interests	2.7	3.4	5.2	4.7
Net (loss) income attributable to ATI	$ (422.6)	$ 75.1	$ (401.5)	$ 90.1
Basic net (loss) income attributable to ATI per common share (in dollars per share)	$ (3.34)	$ 0.60	$ (3.18)	$ 0.72
Diluted net (loss) income attributable to ATI per common share (in dollars per share)	$ (3.34)	$ 0.54	$ (3.18)	$ 0.66