XML 44 R87.htm IDEA: XBRL DOCUMENT v2.4.1.9
Pension Plans and Other Postretirement Benefits (Details 11) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan Fair Value Of Plan Assets $ 2,204.4us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan Fair Value Of Plan Assets 409.5us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Significant Observable Inputs (Level 2) [Member] | Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan Fair Value Of Plan Assets 985.2us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Significant Unobservable Inputs (Level 3) [Member] | Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan Fair Value Of Plan Assets 809.7us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Equity Securities: ATI common stock [Member] | Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan Fair Value Of Plan Assets 102.7us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Defined Benefit Plan Disclosure [Line Items]      
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Defined Benefit Plan Disclosure [Line Items]      
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Equity Securities: ATI common stock [Member] | Significant Unobservable Inputs (Level 3) [Member] | Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
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Equity Securities: Other U.S. equities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Percentage of co-mingled funds comprised of large-cap investments 90.00%ati_DefinedBenefitPlanInvestmentComingledFundsPercentageComprisedofLargecapInvestments
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Percentage of co-mingled funds comprised of small-cap investments 10.00%ati_DefinedBenefitPlanInvestmentComingledFundsPercentageComprisedofSmallcapInvestments
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Equity Securities: Other U.S. equities [Member] | Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan Fair Value Of Plan Assets 673.8us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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Defined Benefit Plan Disclosure [Line Items]      
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Defined Benefit Plan Disclosure [Line Items]      
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Defined Benefit Plan Disclosure [Line Items]      
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Equity Securities: International equities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
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Percentage of co-mingled funds comprised of emerging market investments 10.00%ati_DefinedBenefitPlanInvestmentComingledFundsPercentageComprisedofEmergingMarketInvestments
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Equity Securities: International equities [Member] | Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
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Defined Benefit Plan Disclosure [Line Items]      
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Defined Benefit Plan Disclosure [Line Items]      
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Defined Benefit Plan Disclosure [Line Items]      
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Defined Benefit Plan Disclosure [Line Items]      
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Defined Benefit Plan Disclosure [Line Items]      
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[1] Includes investments in commingled funds that invest in U.S. equity securities, comprised of approximately 90% large-cap U.S. companies and 10% small-cap U.S. companies.
[2] Includes investments in commingled funds that invest in non-U.S. equity securities, comprised of approximately 90% developed countries and 10% emerging market economies.
[3] Includes investments in commingled funds that invest in non-U.S. equity securities, comprised of approximately 80% developed countries and 20% emerging market economies.
[4] Global debt securities include both fixed interest rate and floating interest rate instruments. These are comprised of actively managed investments which include U.S. government and U.S. government agency securities, foreign government securities, corporate bonds, mortgage-backed securities and other debt securities, and include both investment grade and non-investment grade debt, public and private debt, and secured and unsecured debt investments. To mitigate risk, investment managers have limitations regarding the amount of investment in particular securities and the credit quality of such investments.