Consolidated Statements of Operations - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Income Statement [Abstract]
Sales	$ 3,836.0	$ 2,799.8	$ 2,982.1
Costs and expenses:
Cost of sales	3,121.8	2,466.6	2,689.3
Gross profit	714.2	333.2	292.8
Selling and administrative expenses	297.5	226.9	201.0
Impairment of goodwill	0.0	0.0	287.0
Restructuring charges (credits)	(4.8)	(11.3)	1,107.5
Loss on asset sales and sales of businesses, net	134.2	0.0	0.0
Operating income (loss)	287.3	117.6	(1,302.7)
Nonoperating retirement benefit income (expense)	(25.4)	37.2	(62.1)
Interest expense, net	(87.4)	(96.9)	(94.4)
Debt extinguishment charge	0.0	(65.5)	(21.5)
Other income (loss), net	(12.5)	18.2	(1.2)
Income (loss) before income taxes	162.0	10.6	(1,481.9)
Income tax provision	15.5	26.8	77.7
Net income (loss)	146.5	(16.2)	(1,559.6)
Less: Net income attributable to noncontrolling interests	15.6	22.0	13.0
Net income (loss) attributable to ATI	$ 130.9	$ (38.2)	$ (1,572.6)
Basic net income (loss) per common share
Continuing operations attributable to ATI per common share (in dollars per share)	$ 1.03	$ (0.30)	$ (12.43)
Basic net income (loss) attributable to ATI per common share (in dollars per share)	1.03	(0.30)	(12.43)
Diluted net income (loss) per common share
Continuing operations attributable to ATI per common share (in dollars per share)	0.96	(0.30)	(12.43)
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.96	$ (0.30)	$ (12.43)