Financial Information for Subsidiary and Guarantor Parent (I.S.) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement [Abstract]
Sales	$ 832.7	$ 1,069.6	$ 2,980.7	$ 3,175.9
Cost of sales	861.4	972.6	2,822.9	2,919.2
Selling and administrative expenses	62.5	68.7	198.0	202.1
Income (loss) before interest, other income and income taxes	(91.2)	28.3	(40.2)	54.6
Interest income (expense), net	(27.5)	(25.2)	(81.0)	(82.8)
Other income (loss) including equity in income of unconsolidated subsidiaries	0.8	1.0	2.3	2.9
Income (loss) from continuing operations before income tax provision (benefit)	(117.9)	4.1	(118.9)	(25.3)
Income tax provision (benefit)	23.4	0.5	23.7	(12.4)
Income (loss) from continuing operations	(141.3)	3.6	(142.6)	(12.9)
Income (loss) from discontinued operations, net of tax	0.0	(0.7)	0.0	(2.8)
Net income (loss)	(141.3)	2.9	(142.6)	(15.7)
Less: Net income attributable to noncontrolling interests	3.3	3.6	8.4	9.0
Net income (loss) attributable to ATI	(144.6)	(0.7)	(151.0)	(24.7)
Comprehensive income (loss) attributable to ATI	(162.7)	9.9	(145.0)	34.9
Parent Company
Income Statement [Abstract]
Sales	0.0	0.0	0.0	0.0
Cost of sales	9.2	7.8	22.0	34.9
Selling and administrative expenses	22.9	27.5	70.1	75.5
Income (loss) before interest, other income and income taxes	(32.1)	(35.3)	(92.1)	(110.4)
Interest income (expense), net	(29.5)	(26.4)	(86.1)	(83.5)
Other income (loss) including equity in income of unconsolidated subsidiaries	(56.3)	65.8	59.3	168.6
Income (loss) from continuing operations before income tax provision (benefit)	(117.9)	4.1	(118.9)	(25.3)
Income tax provision (benefit)	23.4	0.5	23.7	(12.4)
Income (loss) from continuing operations	(141.3)	3.6	(142.6)	(12.9)
Income (loss) from discontinued operations, net of tax	0.0	(0.7)	0.0	(2.8)
Net income (loss)	(141.3)	2.9	(142.6)	(15.7)
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to ATI	(141.3)	2.9	(142.6)	(15.7)
Comprehensive income (loss) attributable to ATI	(163.5)	15.3	(140.4)	42.7
Subsidiaries
Income Statement [Abstract]
Sales	286.3	473.8	1,231.5	1,415.5
Cost of sales	363.5	465.0	1,296.5	1,429.8
Selling and administrative expenses	9.3	12.2	30.4	32.6
Income (loss) before interest, other income and income taxes	(86.5)	(3.4)	(95.4)	(46.9)
Interest income (expense), net	(12.6)	(11.3)	(37.3)	(33.1)
Other income (loss) including equity in income of unconsolidated subsidiaries	0.2	0.4	0.8	0.9
Income (loss) from continuing operations before income tax provision (benefit)	(98.9)	(14.3)	(131.9)	(79.1)
Income tax provision (benefit)	(35.1)	(4.6)	(46.3)	(27.7)
Income (loss) from continuing operations	(63.8)	(9.7)	(85.6)	(51.4)
Income (loss) from discontinued operations, net of tax	0.0	0.0	0.0	0.0
Net income (loss)	(63.8)	(9.7)	(85.6)	(51.4)
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to ATI	(63.8)	(9.7)	(85.6)	(51.4)
Comprehensive income (loss) attributable to ATI	(60.5)	(7.8)	(76.0)	(45.4)
NonGuarantor Subsidiaries
Income Statement [Abstract]
Sales	546.4	595.8	1,749.2	1,760.4
Cost of sales	488.7	499.8	1,504.4	1,454.5
Selling and administrative expenses	30.3	29.0	97.5	94.0
Income (loss) before interest, other income and income taxes	27.4	67.0	147.3	211.9
Interest income (expense), net	14.6	12.5	42.4	33.8
Other income (loss) including equity in income of unconsolidated subsidiaries	0.7	0.5	1.8	1.9
Income (loss) from continuing operations before income tax provision (benefit)	42.7	80.0	191.5	247.6
Income tax provision (benefit)	11.0	23.4	67.4	88.4
Income (loss) from continuing operations	31.7	56.6	124.1	159.2
Income (loss) from discontinued operations, net of tax	0.0	(0.7)	0.0	(2.8)
Net income (loss)	31.7	55.9	124.1	156.4
Less: Net income attributable to noncontrolling interests	3.3	3.6	8.4	9.0
Net income (loss) attributable to ATI	28.4	52.3	115.7	147.4
Comprehensive income (loss) attributable to ATI	16.7	34.1	101.9	136.8
Consolidation Eliminations
Income Statement [Abstract]
Sales	0.0	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0	0.0
Selling and administrative expenses	0.0	0.0	0.0	0.0
Income (loss) before interest, other income and income taxes	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	56.2	(65.7)	(59.6)	(168.5)
Income (loss) from continuing operations before income tax provision (benefit)	56.2	(65.7)	(59.6)	(168.5)
Income tax provision (benefit)	24.1	(18.8)	(21.1)	(60.7)
Income (loss) from continuing operations	32.1	(46.9)	(38.5)	(107.8)
Income (loss) from discontinued operations, net of tax	0.0	0.7	0.0	2.8
Net income (loss)	32.1	(46.2)	(38.5)	(105.0)
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to ATI	32.1	(46.2)	(38.5)	(105.0)
Comprehensive income (loss) attributable to ATI	$ 44.6	$ (31.7)	$ (30.5)	$ (99.2)