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Financial Information for Subsidiary and Guarantor Parent (Comp Inc.) (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]
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
Currency translation adjustment arising during the period									(47.1)	(37.0)	(33.6)
Net derivative gain on hedge transactions									19.6	(32.0)	25.9
Pension and postretirement benefits									(61.4)	(19.7)	(212.6)
Other comprehensive loss, net of tax									(88.9)	(88.7)	(220.3)
Comprehensive loss									(716.0)	(454.6)	(210.7)
Less: Comprehensive income attributable to noncontrolling interest									4.1	6.4	10.1
Comprehensive loss attributable to ATI									(720.1)	(461.0)	(220.8)
Legal Entities [Member] \| Guarantor Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									(627.1)	(365.9)	9.6
Currency translation adjustment arising during the period									(47.1)	(37.0)	(33.6)
Net derivative gain on hedge transactions									19.6	(32.0)	25.9
Pension and postretirement benefits									(61.4)	(19.7)	(212.6)
Other comprehensive loss, net of tax									(88.9)	(88.7)	(220.3)
Comprehensive loss									(716.0)	(454.6)	(210.7)
Less: Comprehensive income attributable to noncontrolling interest									0.0	0.0	0.0
Comprehensive loss attributable to ATI									(716.0)	(454.6)	(210.7)
Legal Entities [Member] \| Subsidiary [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									(160.0)	(288.1)	(55.3)
Currency translation adjustment arising during the period									0.0	0.0	0.0
Net derivative gain on hedge transactions									0.0	0.0	0.0
Pension and postretirement benefits									17.2	29.8	1.8
Other comprehensive loss, net of tax									17.2	29.8	1.8
Comprehensive loss									(142.8)	(258.3)	(53.5)
Less: Comprehensive income attributable to noncontrolling interest									0.0	0.0	0.0
Comprehensive loss attributable to ATI									(142.8)	(258.3)	(53.5)
Legal Entities [Member] \| Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									(140.2)	158.9	228.9
Currency translation adjustment arising during the period									(47.1)	(37.0)	(33.6)
Net derivative gain on hedge transactions									0.0	0.0	0.0
Pension and postretirement benefits									1.8	3.1	(28.4)
Other comprehensive loss, net of tax									(45.3)	(33.9)	(62.0)
Comprehensive loss									(185.5)	125.0	166.9
Less: Comprehensive income attributable to noncontrolling interest									4.1	6.4	10.1
Comprehensive loss attributable to ATI									(189.6)	118.6	156.8
Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									300.2	129.2	(173.6)
Currency translation adjustment arising during the period									47.1	37.0	33.6
Net derivative gain on hedge transactions									0.0	0.0	0.0
Pension and postretirement benefits									(19.0)	(32.9)	26.6
Other comprehensive loss, net of tax									28.1	4.1	60.2
Comprehensive loss									328.3	133.3	(113.4)
Less: Comprehensive income attributable to noncontrolling interest									0.0	0.0	0.0
Comprehensive loss attributable to ATI									$ 328.3	$ 133.3	$ (113.4)