Financial Information for Subsidiary and Guarantor Parent (I.S.) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Sales	$ 796.1	$ 770.5	$ 810.5	$ 757.5	$ 738.9	$ 832.7	$ 1,022.5	$ 1,125.5	$ 3,134.6	$ 3,719.6	$ 4,223.4
Cost of sales									2,972.1	3,659.3	3,844.8
Gross profit	97.3	50.2	48.2	(33.2)	(97.5)	(28.7)	77.0	109.5	162.5	60.3	378.6
Selling and administrative expenses									247.7	238.8	272.5
Restructuring charges									527.2	64.3	0.0
Impairment of goodwill									0.0	126.6	0.0
Restructuring costs									48.8
Operating income (loss)									(612.4)	(369.4)	106.1
Interest expense, net									(124.0)	(110.2)	(108.7)
Other income (expense) including equity in income of unconsolidated subsidiaries									2.4	1.6	4.1
Income (loss) from continuing operations before income taxes									(734.0)	(478.0)	1.5
Income tax benefit									(106.9)	(112.1)	(8.7)
Income (loss) from continuing operations									(627.1)	(365.9)	10.2
Income (loss) from discontinued operations, net of tax									0.0	0.0	(0.6)
Net income (loss)	13.7	(527.2)	(15.5)	(98.1)	(223.3)	(141.3)	(13.9)	12.6	(627.1)	(365.9)	9.6
Less: Net income attributable to noncontrolling interests									13.8	12.0	12.2
Net loss attributable to ATI	$ 9.9	$ (530.8)	$ (18.8)	$ (101.2)	$ (226.9)	$ (144.6)	$ (16.4)	$ 10.0	(640.9)	(377.9)	(2.6)
Legal Entities [Member] | Guarantor Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Sales									0.0	0.0	0.0
Cost of sales									53.4	27.3	45.3
Gross profit									(53.4)	(27.3)	(45.3)
Selling and administrative expenses									89.9	88.2	103.9
Restructuring charges									0.0	1.6
Impairment of goodwill										0.0
Operating income (loss)									(143.3)	(117.1)	(149.2)
Interest expense, net									(138.3)	(117.3)	(111.0)
Other income (expense) including equity in income of unconsolidated subsidiaries									(452.4)	(243.6)	261.7
Income (loss) from continuing operations before income taxes									(734.0)	(478.0)	1.5
Income tax benefit									(106.9)	(112.1)	(8.7)
Income (loss) from continuing operations											10.2
Income (loss) from discontinued operations, net of tax											(0.6)
Net income (loss)									(627.1)	(365.9)	9.6
Less: Net income attributable to noncontrolling interests									0.0	0.0	0.0
Net loss attributable to ATI									(627.1)	(365.9)	9.6
Legal Entities [Member] | Subsidiary [Member]
Condensed Financial Statements, Captions [Line Items]
Sales									983.6	1,453.2	1,878.0
Cost of sales									1,102.0	1,643.2	1,874.8
Gross profit									(118.4)	(190.0)	3.2
Selling and administrative expenses									32.3	29.4	44.0
Restructuring charges									25.1	58.0
Impairment of goodwill										126.6
Operating income (loss)									(175.8)	(404.0)	(40.8)
Interest expense, net									(70.7)	(50.9)	(44.9)
Other income (expense) including equity in income of unconsolidated subsidiaries									1.0	1.1	1.1
Income (loss) from continuing operations before income taxes									(245.5)	(453.8)	(84.6)
Income tax benefit									(85.5)	(165.7)	(29.3)
Income (loss) from continuing operations											(55.3)
Income (loss) from discontinued operations, net of tax											0.0
Net income (loss)									(160.0)	(288.1)	(55.3)
Less: Net income attributable to noncontrolling interests									0.0	0.0	0.0
Net loss attributable to ATI									(160.0)	(288.1)	(55.3)
Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Sales									2,151.0	2,266.4	2,345.4
Cost of sales									1,816.7	1,988.8	1,924.7
Gross profit									334.3	277.6	420.7
Selling and administrative expenses									125.5	121.2	124.6
Restructuring charges									502.1	4.7
Impairment of goodwill										0.0
Operating income (loss)									(293.3)	151.7	296.1
Interest expense, net									85.0	58.0	47.2
Other income (expense) including equity in income of unconsolidated subsidiaries									1.4	0.8	2.9
Income (loss) from continuing operations before income taxes									(206.9)	210.5	346.2
Income tax benefit									(66.7)	51.6	116.7
Income (loss) from continuing operations											229.5
Income (loss) from discontinued operations, net of tax											(0.6)
Net income (loss)									(140.2)	158.9	228.9
Less: Net income attributable to noncontrolling interests									13.8	12.0	12.2
Net loss attributable to ATI									(154.0)	146.9	216.7
Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Sales									0.0	0.0	0.0
Cost of sales									0.0	0.0	0.0
Gross profit									0.0	0.0	0.0
Selling and administrative expenses									0.0	0.0	0.0
Restructuring charges									0.0	0.0
Impairment of goodwill										0.0
Operating income (loss)									0.0	0.0	0.0
Interest expense, net									0.0	0.0	0.0
Other income (expense) including equity in income of unconsolidated subsidiaries									452.4	243.3	(261.6)
Income (loss) from continuing operations before income taxes									452.4	243.3	(261.6)
Income tax benefit									152.2	114.1	(87.4)
Income (loss) from continuing operations											(174.2)
Income (loss) from discontinued operations, net of tax											0.6
Net income (loss)									300.2	129.2	(173.6)
Less: Net income attributable to noncontrolling interests									0.0	0.0	0.0
Net loss attributable to ATI									$ 300.2	$ 129.2	$ (173.6)