CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Sales	$ 1,018.7	$ 1,020.2	$ 3,103.9	$ 3,008.7
Cost of sales	859.0	859.8	2,635.4	2,526.0
Gross profit	159.7	160.4	468.5	482.7
Selling and administrative expenses	65.2	65.5	200.9	195.3
Operating income	94.5	94.9	267.6	287.4
Nonoperating retirement benefit expense	(18.5)	(8.4)	(55.2)	(25.5)
Interest expense, net	(24.2)	(24.8)	(74.9)	(75.8)
Other income, net	67.2	0.8	82.9	22.4
Income before income taxes	119.0	62.5	220.4	208.5
Income tax provision	3.7	6.9	10.3	16.8
Net income	115.3	55.6	210.1	191.7
Less: Net income attributable to noncontrolling interests	4.3	5.1	9.0	10.4
Net income attributable to ATI	$ 111.0	$ 50.5	$ 201.1	$ 181.3
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.88	$ 0.40	$ 1.60	$ 1.45
Diluted net income attributable to ATI per common share (in dollars per share)	$ 0.78	$ 0.37	$ 1.44	$ 1.31