CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Sales	$ 1,009.5	$ 880.2	$ 1,988.5	$ 1,746.1
Cost of sales	835.8	755.9	1,666.2	1,497.0
Gross profit	173.7	124.3	322.3	249.1
Selling and administrative expenses	62.7	65.0	129.8	122.9
Operating income	111.0	59.3	192.5	126.2
Nonoperating retirement benefit expense	(8.8)	(13.7)	(17.1)	(27.3)
Interest expense, net	(25.5)	(34.5)	(51.0)	(68.0)
Other income, net	3.8	0.2	21.6	3.5
Income before income taxes	80.5	11.3	146.0	34.4
Income tax provision (benefit)	4.9	(2.1)	9.9	(0.1)
Net income (loss)	75.6	13.4	136.1	34.5
Less: Net income attributable to noncontrolling interests	2.8	3.3	5.3	6.9
Net income (loss) attributable to ATI	$ 72.8	$ 10.1	$ 130.8	$ 27.6
Basic net income attributable to ATI per common share (in dollars per share)	$ 0.58	$ 0.09	$ 1.05	$ 0.26
Diluted net income attributable to ATI per common share (in dollars per share)	0.52	0.09	0.94	0.25
Dividends declared per common share (in dollars per share)	$ 0	$ 0	$ 0	$ 0