CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Sales	$ 1,020.2	$ 869.1	$ 3,008.7	$ 2,615.2
Cost of sales	859.8	763.8	2,526.0	2,260.8
Gross profit	160.4	105.3	482.7	354.4
Selling and administrative expenses	65.5	64.6	195.3	187.5
Impairment of goodwill	0.0	114.4	0.0	114.4
Operating income (loss)	94.9	(73.7)	287.4	52.5
Nonoperating retirement benefit expense	(8.4)	(13.6)	(25.5)	(40.9)
Interest expense, net	(24.8)	(34.2)	(75.8)	(102.2)
Other income, net	0.8	0.2	22.4	3.7
Income (loss) before income taxes	62.5	(121.3)	208.5	(86.9)
Income tax provision (benefit)	6.9	(1.9)	16.8	(2.0)
Net income (loss)	55.6	(119.4)	191.7	(84.9)
Less: Net income attributable to noncontrolling interests	5.1	1.8	10.4	8.7
Net income (loss) attributable to ATI	$ 50.5	$ (121.2)	$ 181.3	$ (93.6)
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.40	$ (1.12)	$ 1.45	$ (0.87)
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	0.37	(1.12)	1.31	(0.87)
Dividends declared per common share (in dollars per share)	$ 0	$ 0	$ 0	$ 0