Consolidated Statements of Operations - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Sales	$ 2,982.1	$ 4,122.5	$ 4,046.6
Costs and expenses:
Cost of sales	2,689.3	3,484.7	3,416.3
Gross profit	292.8	637.8	630.3
Selling and administrative expenses	201.0	267.2	268.2
Impairment of goodwill	287.0	0.0	0.0
Restructuring charges	1,107.5	4.5	0.0
Operating income (loss)	(1,302.7)	366.1	362.1
Nonoperating retirement benefit expense	(62.1)	(73.6)	(33.9)
Interest expense, net	(94.4)	(99.0)	(101.0)
Debt extinguishment charge	(21.5)	(21.6)	0.0
Other income (loss), net	(1.2)	69.7	20.5
Income (loss) before income taxes	(1,481.9)	241.6	247.7
Income tax provision (benefit)	77.7	(28.5)	11.0
Net income (loss)	(1,559.6)	270.1	236.7
Less: Net income attributable to noncontrolling interests	13.0	12.5	14.3
Net income (loss) attributable to ATI	$ (1,572.6)	$ 257.6	$ 222.4
Basic net income (loss) per common share
Continuing operations attributable to ATI per common share (in dollars per share)	$ (12.43)	$ 2.05	$ 1.78
Basic net income (loss) attributable to ATI per common share (in dollars per share)	(12.43)	2.05	1.78
Diluted net income (loss) per common share
Continuing operations attributable to ATI per common share (in dollars per share)	(12.43)	1.85	1.61
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ (12.43)	$ 1.85	$ 1.61