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Financial Information for Subsidiary and Guarantor Parent (Comp Inc.) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	$ 25.3us-gaap_ProfitLoss	$ 2.9us-gaap_ProfitLoss	$ (0.7)us-gaap_ProfitLoss	$ (17.9)us-gaap_ProfitLoss	$ 175.6us-gaap_ProfitLoss	$ (32.2)us-gaap_ProfitLoss	$ 6.6us-gaap_ProfitLoss	$ 11.6us-gaap_ProfitLoss	$ 9.6us-gaap_ProfitLoss	$ 161.6us-gaap_ProfitLoss	$ 167.8us-gaap_ProfitLoss
Currency translation adjustment arising during the period									(33.6)	15.3	14.3
Unrealized holding gain (loss) on securities									0us-gaap_UnrealizedGainLossOnSecurities	0.1us-gaap_UnrealizedGainLossOnSecurities	0us-gaap_UnrealizedGainLossOnSecurities
Unrealized holding gain (loss) on securities									0	0.1	0
Net derivative gain (loss) on hedge transactions									25.9	(6.9)	(2.8)
Pension and postretirement benefits									(212.6)	311.1	(97.4)
Other comprehensive income (loss), net of tax									(220.3)	319.6	(85.9)
Comprehensive income (loss)									(210.7)	481.2	81.9
Less: Comprehensive income attributable to noncontrolling interest									10.1	11.0	11.3
Comprehensive income (loss) attributable to ATI									(220.8)us-gaap_ComprehensiveIncomeNetOfTax	470.2us-gaap_ComprehensiveIncomeNetOfTax	70.6us-gaap_ComprehensiveIncomeNetOfTax
Guarantor Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									9.6	161.6	167.8
Currency translation adjustment arising during the period									(33.6)	15.3	14.3
Unrealized holding gain (loss) on securities									0	0.1	0
Net derivative gain (loss) on hedge transactions									25.9	(6.9)	(2.8)
Pension and postretirement benefits									(212.6)	311.1	(97.4)
Other comprehensive income (loss), net of tax									(220.3)	319.6	(85.9)
Comprehensive income (loss)									(210.7)	481.2	81.9
Less: Comprehensive income attributable to noncontrolling interest									0	0	0
Comprehensive income (loss) attributable to ATI									(210.7)	481.2	81.9
Subsidiary [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									(55.3)	(46.3)	41.5
Currency translation adjustment arising during the period									0	0	0
Unrealized holding gain (loss) on securities									0	0	0
Net derivative gain (loss) on hedge transactions									0	0	0
Pension and postretirement benefits									1.8	22.0	(18.1)
Other comprehensive income (loss), net of tax									1.8	22.0	(18.1)
Comprehensive income (loss)									(53.5)	(24.3)	23.4
Less: Comprehensive income attributable to noncontrolling interest									0	0	0
Comprehensive income (loss) attributable to ATI									(53.5)	(24.3)	23.4
Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									228.9	387.9	274.7
Currency translation adjustment arising during the period									(33.6)	15.3	14.3
Unrealized holding gain (loss) on securities									0	0.1	0
Net derivative gain (loss) on hedge transactions									0	0	0
Pension and postretirement benefits									(28.4)	27.6	(5.1)
Other comprehensive income (loss), net of tax									(62.0)	43.0	9.2
Comprehensive income (loss)									166.9	430.9	283.9
Less: Comprehensive income attributable to noncontrolling interest									10.1	11.0	11.3
Comprehensive income (loss) attributable to ATI									156.8	419.9	272.6
Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									(173.6)	(341.6)	(316.2)
Currency translation adjustment arising during the period									33.6	(15.3)	(14.3)
Unrealized holding gain (loss) on securities									0	(0.1)	0
Net derivative gain (loss) on hedge transactions									0	0	0
Pension and postretirement benefits									26.6	(49.6)	23.2
Other comprehensive income (loss), net of tax									60.2	(65.0)	8.9
Comprehensive income (loss)									(113.4)	(406.6)	(307.3)
Less: Comprehensive income attributable to noncontrolling interest									0	0	0
Comprehensive income (loss) attributable to ATI									$ (113.4)	$ (406.6)	$ (307.3)