Consolidated Statements of Operations - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Sales	$ 1,037.9	$ 1,020.2	$ 1,009.5	$ 979.0	$ 909.9	$ 869.1	$ 880.2	$ 865.9	$ 4,046.6	$ 3,525.1	$ 3,134.6
Costs and expenses:
Cost of sales									3,416.3	3,028.1	2,911.8
Gross profit	147.6	160.4	173.7	148.6	142.6	105.3	124.3	124.8	630.3	497.0	222.8
Selling and administrative expenses									268.2	248.0	240.8
Impairment of goodwill						114.4			0.0	114.4	0.0
Restructuring charges									0.0	0.0	523.8
Operating income (loss)									362.1	134.6	(541.8)
Nonoperating retirement benefit expense									(33.9)	(54.3)	(70.6)
Interest expense, net									(101.0)	(133.8)	(124.0)
Debt extinguishment charge					(37.0)				0.0	(37.0)	0.0
Other income, net									20.5	4.0	2.4
Income (loss) before income taxes									247.7	(86.5)	(734.0)
Income tax provision (benefit)									11.0	(6.8)	(106.9)
Net income (loss)	45.0	55.6	75.6	60.5	5.2	(119.4)	13.4	21.1	236.7	(79.7)	(627.1)
Less: Net income attributable to noncontrolling interests									14.3	12.2	13.8
Net income (loss) attributable to ATI	$ 41.1	$ 50.5	$ 72.8	$ 58.0	$ 1.7	$ (121.2)	$ 10.1	$ 17.5	$ 222.4	$ (91.9)	$ (640.9)
Basic net income (loss) per common share
Continuing operations attributable to ATI per common share (in dollars per share)									$ 1.78	$ (0.83)	$ (5.97)
Basic net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.33	$ 0.40	$ 0.58	$ 0.46	$ 0.01	$ (1.12)	$ 0.09	$ 0.16	1.78	(0.83)	(5.97)
Diluted net income (loss) per common share
Continuing operations attributable to ATI per common share (in dollars per share)									1.61	(0.83)	(5.97)
Diluted net income (loss) attributable to ATI per common share (in dollars per share)	$ 0.30	$ 0.37	$ 0.52	$ 0.42	$ 0.01	$ (1.12)	$ 0.09	$ 0.16	$ 1.61	$ (0.83)	$ (5.97)