Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Sales	$ 3,836.0	$ 2,799.8	$ 2,982.1
Gross profit	714.2	333.2	292.8
Net income (loss)	$ 146.5	$ (16.2)	$ (1,559.6)
Basic income (loss) 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 income (loss) 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)
Average shares outstanding (shares)	127.5	127.1	126.5