Financial Information for Subsidiary and Guarantor Parent (I.S.) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
Sales	$ 770.5	$ 832.7	$ 2,338.5	$ 2,980.7
Cost of sales	720.3	861.4	2,273.3	2,822.9
Gross profit (loss)	50.2	(28.7)	65.2	157.8
Selling and administrative expenses	60.5	62.5	182.4	198.0
Restructuring charges	488.6	0.0	498.6	0.0
Operating income (loss)	(498.9)	(91.2)	(615.8)	(40.2)
Interest income (expense), net	(32.6)	(27.5)	(91.2)	(81.0)
Other income (loss) including equity in income of unconsolidated subsidiaries	0.0	0.8	1.8	2.3
Income (loss) before income tax provision (benefit)	(531.5)	(117.9)	(705.2)	(118.9)
Income tax provision (benefit)	(4.3)	23.4	(64.4)	23.7
Net income (loss)	(527.2)	(141.3)	(640.8)	(142.6)
Less: Net income attributable to noncontrolling interests	3.6	3.3	10.0	8.4
Net income (loss) attributable to ATI	(530.8)	(144.6)	(650.8)	(151.0)
Comprehensive income (loss) attributable to ATI	(518.8)	(162.7)	(598.0)	(145.0)
Reportable Legal Entities | Parent Company
Income Statement [Abstract]
Sales	0.0	0.0	0.0	0.0
Cost of sales	14.0	9.2	43.0	22.0
Gross profit (loss)	(14.0)	(9.2)	(43.0)	(22.0)
Selling and administrative expenses	23.8	22.9	66.3	70.1
Restructuring charges	0.0		0.0
Operating income (loss)	(37.8)	(32.1)	(109.3)	(92.1)
Interest income (expense), net	(36.5)	(29.5)	(101.4)	(86.1)
Other income (loss) including equity in income of unconsolidated subsidiaries	(457.2)	(56.3)	(494.5)	59.3
Income (loss) before income tax provision (benefit)	(531.5)	(117.9)	(705.2)	(118.9)
Income tax provision (benefit)	(4.3)	23.4	(64.4)	23.7
Net income (loss)	(527.2)	(141.3)	(640.8)	(142.6)
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to ATI	(527.2)	(141.3)	(640.8)	(142.6)
Comprehensive income (loss) attributable to ATI	(516.8)	(163.5)	(592.6)	(140.4)
Reportable Legal Entities | Subsidiary
Income Statement [Abstract]
Sales	253.0	286.3	727.7	1,231.5
Cost of sales	266.6	363.5	852.0	1,296.5
Gross profit (loss)	(13.6)	(77.2)	(124.3)	(65.0)
Selling and administrative expenses	4.2	9.3	18.8	30.4
Restructuring charges	0.5		9.5
Operating income (loss)	(18.3)	(86.5)	(152.6)	(95.4)
Interest income (expense), net	(19.3)	(12.6)	(51.0)	(37.3)
Other income (loss) including equity in income of unconsolidated subsidiaries	0.2	0.2	0.6	0.8
Income (loss) before income tax provision (benefit)	(37.4)	(98.9)	(203.0)	(131.9)
Income tax provision (benefit)	(9.1)	(35.1)	(70.1)	(46.3)
Net income (loss)	(28.3)	(63.8)	(132.9)	(85.6)
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to ATI	(28.3)	(63.8)	(132.9)	(85.6)
Comprehensive income (loss) attributable to ATI	(27.0)	(60.5)	(114.2)	(76.0)
Reportable Legal Entities | NonGuarantor Subsidiaries
Income Statement [Abstract]
Sales	517.5	546.4	1,610.8	1,749.2
Cost of sales	439.7	488.7	1,378.3	1,504.4
Gross profit (loss)	77.8	57.7	232.5	244.8
Selling and administrative expenses	32.5	30.3	97.3	97.5
Restructuring charges	488.1		489.1
Operating income (loss)	(442.8)	27.4	(353.9)	147.3
Interest income (expense), net	23.2	14.6	61.2	42.4
Other income (loss) including equity in income of unconsolidated subsidiaries	(0.2)	0.7	1.5	1.8
Income (loss) before income tax provision (benefit)	(419.8)	42.7	(291.2)	191.5
Income tax provision (benefit)	(165.8)	11.0	(116.3)	67.4
Net income (loss)	(254.0)	31.7	(174.9)	124.1
Less: Net income attributable to noncontrolling interests	3.6	3.3	10.0	8.4
Net income (loss) attributable to ATI	(257.6)	28.4	(184.9)	115.7
Comprehensive income (loss) attributable to ATI	(260.0)	16.7	(199.0)	101.9
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
Restructuring charges	0.0		0.0
Operating income (loss)	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	457.2	56.2	494.2	(59.6)
Income (loss) before income tax provision (benefit)	457.2	56.2	494.2	(59.6)
Income tax provision (benefit)	174.9	24.1	186.4	(21.1)
Net income (loss)	282.3	32.1	307.8	(38.5)
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to ATI	282.3	32.1	307.8	(38.5)
Comprehensive income (loss) attributable to ATI	$ 285.0	$ 44.6	$ 307.8	$ (30.5)