CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Sales	$ 1,080.4	$ 1,009.5	$ 2,085.2	$ 1,988.5
Cost of sales	902.7	835.8	1,776.4	1,666.2
Gross profit	177.7	173.7	308.8	322.3
Selling and administrative expenses	67.7	62.7	135.7	129.8
Operating income	110.0	111.0	173.1	192.5
Nonoperating retirement benefit expense	(18.4)	(8.8)	(36.7)	(17.1)
Interest expense, net	(25.9)	(25.5)	(50.7)	(51.0)
Other income, net	18.6	3.8	15.7	21.6
Income before income taxes	84.3	80.5	101.4	146.0
Income tax provision	5.8	4.9	6.6	9.9
Net income	78.5	75.6	94.8	136.1
Less: Net income attributable to noncontrolling interests	3.4	2.8	4.7	5.3
Net income attributable to ATI	$ 75.1	$ 72.8	$ 90.1	$ 130.8
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.60	$ 0.58	$ 0.72	$ 1.05
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.54	$ 0.52	$ 0.66	$ 0.94