Financial Information for Subsidiary and Guarantor Parent (I.S.) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statement [Abstract]
Sales	$ 1,022.5	$ 1,119.0	$ 2,148.0	$ 2,106.3
Cost of sales	945.5	1,029.5	1,961.5	1,946.6
Selling and administrative expenses	72.4	65.7	135.5	133.4
Income (loss) before interest, other income and income taxes	4.6	23.8	51.0	26.3
Interest income (expense), net	(26.8)	(28.5)	(53.5)	(57.6)
Other income (loss) including equity in income of unconsolidated subsidiaries	0.6	1.3	1.5	1.9
Income (loss) from continuing operations before income tax provision (benefit)	(21.6)	(3.4)	(1.0)	(29.4)
Income tax provision (benefit)	(7.7)	(2.9)	0.3	(12.9)
Loss from continuing operations	(13.9)	(0.5)	(1.3)	(16.5)
Income (loss) from discontinued operations, net of tax	0.0	(0.2)	0.0	(2.1)
Net income (loss)	(13.9)	(0.7)	(1.3)	(18.6)
Less: Net income attributable to noncontrolling interests	2.5	3.3	5.1	5.4
Net income (loss) attributable to ATI	(16.4)	(4.0)	(6.4)	(24.0)
Comprehensive income (loss) attributable to ATI	7.1	26.0	17.7	25.0
Parent Company
Income Statement [Abstract]
Sales	0.0	0.0	0.0	0.0
Cost of sales	10.6	14.8	12.8	27.1
Selling and administrative expenses	21.4	20.9	47.2	48.0
Income (loss) before interest, other income and income taxes	(32.0)	(35.7)	(60.0)	(75.1)
Interest income (expense), net	(28.6)	(28.6)	(56.6)	(57.1)
Other income (loss) including equity in income of unconsolidated subsidiaries	39.0	60.9	115.6	102.8
Income (loss) from continuing operations before income tax provision (benefit)	(21.6)	(3.4)	(1.0)	(29.4)
Income tax provision (benefit)	(7.7)	(2.9)	0.3	(12.9)
Loss from continuing operations	(13.9)	(0.5)	(1.3)	(16.5)
Income (loss) from discontinued operations, net of tax	0.0	(0.2)	0.0	(2.1)
Net income (loss)	(13.9)	(0.7)	(1.3)	(18.6)
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to ATI	(13.9)	(0.7)	(1.3)	(18.6)
Comprehensive income (loss) attributable to ATI	10.1	28.2	23.1	27.4
Subsidiaries
Income Statement [Abstract]
Sales	436.7	513.8	945.2	941.7
Cost of sales	429.0	525.2	933.0	964.8
Selling and administrative expenses	9.4	10.3	21.1	20.4
Income (loss) before interest, other income and income taxes	(1.7)	(21.7)	(8.9)	(43.5)
Interest income (expense), net	(12.5)	(11.2)	(24.7)	(21.8)
Other income (loss) including equity in income of unconsolidated subsidiaries	0.2	0.2	0.6	0.5
Income (loss) from continuing operations before income tax provision (benefit)	(14.0)	(32.7)	(33.0)	(64.8)
Income tax provision (benefit)	(4.6)	(11.8)	(11.2)	(23.1)
Loss from continuing operations	(9.4)	(20.9)	(21.8)	(41.7)
Income (loss) from discontinued operations, net of tax	0.0	0.0	0.0	0.0
Net income (loss)	(9.4)	(20.9)	(21.8)	(41.7)
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to ATI	(9.4)	(20.9)	(21.8)	(41.7)
Comprehensive income (loss) attributable to ATI	(6.4)	(18.8)	(15.5)	(37.6)
NonGuarantor Subsidiaries
Income Statement [Abstract]
Sales	585.8	605.2	1,202.8	1,164.6
Cost of sales	505.9	489.5	1,015.7	954.7
Selling and administrative expenses	41.6	34.5	67.2	65.0
Income (loss) before interest, other income and income taxes	38.3	81.2	119.9	144.9
Interest income (expense), net	14.3	11.3	27.8	21.3
Other income (loss) including equity in income of unconsolidated subsidiaries	0.5	0.8	1.1	1.4
Income (loss) from continuing operations before income tax provision (benefit)	53.1	93.3	148.8	167.6
Income tax provision (benefit)	22.3	38.6	56.4	65.0
Loss from continuing operations	30.8	54.7	92.4	102.6
Income (loss) from discontinued operations, net of tax	0.0	(0.2)	0.0	(2.1)
Net income (loss)	30.8	54.5	92.4	100.5
Less: Net income attributable to noncontrolling interests	2.5	3.3	5.1	5.4
Net income (loss) attributable to ATI	28.3	51.2	87.3	95.1
Comprehensive income (loss) attributable to ATI	47.4	60.2	85.2	102.7
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	(39.1)	(60.6)	(115.8)	(102.8)
Income (loss) from continuing operations before income tax provision (benefit)	(39.1)	(60.6)	(115.8)	(102.8)
Income tax provision (benefit)	(17.7)	(26.8)	(45.2)	(41.9)
Loss from continuing operations	(21.4)	(33.8)	(70.6)	(60.9)
Income (loss) from discontinued operations, net of tax	0.0	0.2	0.0	2.1
Net income (loss)	(21.4)	(33.6)	(70.6)	(58.8)
Less: Net income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Net income (loss) attributable to ATI	(21.4)	(33.6)	(70.6)	(58.8)
Comprehensive income (loss) attributable to ATI	$ (44.0)	$ (43.6)	$ (75.1)	$ (67.5)