CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement [Abstract]
Sales	$ 616.2	$ 770.3	$ 1,308.7	$ 1,725.8
Cost of sales	573.5	695.6	1,180.2	1,516.3
Gross profit	42.7	74.7	128.5	209.5
Selling and administrative expenses	60.2	44.4	114.2	102.8
Impairment of goodwill	0.0	287.0	0.0	287.0
Restructuring charges (credits)	(6.2)	16.7	(6.2)	24.7
Operating income (loss)	(11.3)	(273.4)	20.5	(205.0)
Nonoperating retirement benefit expense	(6.8)	(11.2)	(13.6)	(22.4)
Interest expense, net	(23.7)	(21.7)	(47.1)	(43.6)
Debt extinguishment charge (See Note 7)	0.0	(21.5)	0.0	(21.5)
Other income (expense), net	1.4	0.5	2.9	(0.4)
Loss before income taxes	(40.4)	(327.3)	(37.3)	(292.9)
Income tax provision	4.0	92.6	9.5	103.4
Net loss	(44.4)	(419.9)	(46.8)	(396.3)
Less: Net income attributable to noncontrolling interests	4.8	2.7	10.3	5.2
Net loss attributable to ATI	$ (49.2)	$ (422.6)	$ (57.1)	$ (401.5)
Basic net loss attributable to ATI per common share (in dollars per share)	$ (0.39)	$ (3.34)	$ (0.45)	$ (3.18)
Diluted net loss attributable to ATI per common share (in dollars per share)	$ (0.39)	$ (3.34)	$ (0.45)	$ (3.18)