Financial Information for Subsidiary and Guarantor Parent (I.S.) (Details) - 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 (loss)	173.7	124.3	322.3	249.1
Selling and administrative expenses	62.7	65.0	129.8	122.9
Operating income (loss)	111.0	59.3	192.5	126.2
Nonoperating retirement benefit expense	(8.8)	(13.7)	(17.1)	(27.3)
Interest income (expense), net	(25.5)	(34.5)	(51.0)	(68.0)
Other income (loss) including equity in income of unconsolidated subsidiaries	3.8	0.2	21.6	3.5
Income (loss) before income tax provision (benefit)	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
Comprehensive income (loss) attributable to ATI	73.4	14.3	167.0	54.1
Reportable Legal Entities | Parent Company
Income Statement [Abstract]
Sales	0.0	0.0	0.0	0.0
Cost of sales	3.9	5.1	8.0	9.1
Gross profit (loss)	(3.9)	(5.1)	(8.0)	(9.1)
Selling and administrative expenses	24.4	21.9	48.1	42.0
Operating income (loss)	(28.3)	(27.0)	(56.1)	(51.1)
Nonoperating retirement benefit expense	(3.5)	(8.1)	(6.6)	(16.2)
Interest income (expense), net	(34.2)	(39.7)	(67.2)	(78.3)
Other income (loss) including equity in income of unconsolidated subsidiaries	146.5	86.1	275.9	180.0
Income (loss) before income tax provision (benefit)	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	0.0	0.0	0.0	0.0
Net income (loss) attributable to ATI	75.6	13.4	136.1	34.5
Comprehensive income (loss) attributable to ATI	73.1	17.8	176.1	63.2
Reportable Legal Entities | Subsidiary
Income Statement [Abstract]
Sales	343.2	291.2	689.6	578.4
Cost of sales	323.3	274.1	646.5	529.7
Gross profit (loss)	19.9	17.1	43.1	48.7
Selling and administrative expenses	7.6	8.6	17.9	18.1
Operating income (loss)	12.3	8.5	25.2	30.6
Nonoperating retirement benefit expense	(4.8)	(4.6)	(9.7)	(9.3)
Interest income (expense), net	(28.1)	(22.5)	(53.8)	(43.7)
Other income (loss) including equity in income of unconsolidated subsidiaries	2.3	0.3	19.1	0.8
Income (loss) before income tax provision (benefit)	(18.3)	(18.3)	(19.2)	(21.6)
Income tax provision (benefit)	(4.5)	(6.6)	(4.3)	(7.7)
Net income (loss)	(13.8)	(11.7)	(14.9)	(13.9)
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to ATI	(13.8)	(11.7)	(14.9)	(13.9)
Comprehensive income (loss) attributable to ATI	(11.5)	(11.3)	(10.2)	(11.6)
Reportable Legal Entities | NonGuarantor Subsidiaries
Income Statement [Abstract]
Sales	666.3	589.0	1,298.9	1,167.7
Cost of sales	508.6	476.7	1,011.7	958.2
Gross profit (loss)	157.7	112.3	287.2	209.5
Selling and administrative expenses	30.7	34.5	63.8	62.8
Operating income (loss)	127.0	77.8	223.4	146.7
Nonoperating retirement benefit expense	(0.5)	(1.0)	(0.8)	(1.8)
Interest income (expense), net	36.8	27.7	70.0	54.0
Other income (loss) including equity in income of unconsolidated subsidiaries	0.2	(0.1)	0.7	2.7
Income (loss) before income tax provision (benefit)	163.5	104.4	293.3	201.6
Income tax provision (benefit)	24.9	33.6	45.1	69.1
Net income (loss)	138.6	70.8	248.2	132.5
Less: Net income attributable to noncontrolling interests	2.8	3.3	5.3	6.9
Net income (loss) attributable to ATI	135.8	67.5	242.9	125.6
Comprehensive income (loss) attributable to ATI	112.8	92.5	236.8	138.4
Eliminations
Income Statement [Abstract]
Sales	0.0	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0	0.0
Gross profit (loss)	0.0	0.0	0.0	0.0
Selling and administrative expenses	0.0	0.0	0.0	0.0
Operating income (loss)	0.0	0.0	0.0	0.0
Nonoperating retirement benefit expense	0.0	0.0	0.0	0.0
Interest income (expense), net	0.0	0.0	0.0	0.0
Other income (loss) including equity in income of unconsolidated subsidiaries	(145.2)	(86.1)	(274.1)	(180.0)
Income (loss) before income tax provision (benefit)	(145.2)	(86.1)	(274.1)	(180.0)
Income tax provision (benefit)	(20.4)	(27.0)	(40.8)	(61.4)
Net income (loss)	(124.8)	(59.1)	(233.3)	(118.6)
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to ATI	(124.8)	(59.1)	(233.3)	(118.6)
Comprehensive income (loss) attributable to ATI	$ (101.0)	$ (84.7)	$ (235.7)	$ (135.9)