Consolidated Statements of Operations - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Sales	$ 2,799.8	$ 2,982.1	$ 4,122.5
Costs and expenses:
Cost of sales	2,466.6	2,689.3	3,484.5
Gross profit	333.2	292.8	638.0
Selling and administrative expenses	226.9	201.0	267.2
Impairment of goodwill	0.0	287.0	0.0
Restructuring charges (credits)	(11.3)	1,107.5	4.5
Operating income (loss)	117.6	(1,302.7)	366.3
Nonoperating retirement benefit income (expense)	37.2	(62.1)	(73.6)
Interest expense, net	(96.9)	(94.4)	(99.0)
Debt extinguishment charge	(65.5)	(21.5)	(21.6)
Other income (loss), net	18.2	(1.2)	64.4
Income (loss) before income taxes	10.6	(1,481.9)	236.5
Income tax provision (benefit)	26.8	77.7	(28.5)
Net income (loss)	(16.2)	(1,559.6)	265.0
Less: Net income attributable to noncontrolling interests	22.0	13.0	12.5
Net income (loss) attributable to ATI	$ (38.2)	$ (1,572.6)	$ 252.5
Basic net income (loss) per common share
Continuing operations attributable to ATI per common share (in dollars per share)	$ (0.30)	$ (12.43)	$ 2.01
Basic net income (loss) attributable to ATI per common share (in dollars per share)	(0.30)	(12.43)	2.01
Diluted net income (loss) per common share
Continuing operations attributable to ATI per common share (in dollars per share)	(0.30)	(12.43)	1.81
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ (0.30)	$ (12.43)	$ 1.81