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Asset Retirement Obligations (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Loss Contingencies [Line Items]      
Asset retirement obligation $ 25.4us-gaap_AssetRetirementObligation $ 27.7us-gaap_AssetRetirementObligation $ 13.0us-gaap_AssetRetirementObligation
Revision of estimates (1.0)us-gaap_AssetRetirementObligationRevisionOfEstimate 13.8us-gaap_AssetRetirementObligationRevisionOfEstimate  
Tungsten Materials Business [Member]      
Loss Contingencies [Line Items]      
Asset retirement obligation   13.0us-gaap_AssetRetirementObligation
/ us-gaap_StatementBusinessSegmentsAxis
= ati_TungstenMaterialsBusinessMember		  
Asset retirement obligation period   5 years  
Revision of estimates   9.4us-gaap_AssetRetirementObligationRevisionOfEstimate
/ us-gaap_StatementBusinessSegmentsAxis
= ati_TungstenMaterialsBusinessMember		  
Restructuring Charges [Member]      
Loss Contingencies [Line Items]      
Revision of estimates   $ 4.2us-gaap_AssetRetirementObligationRevisionOfEstimate
/ us-gaap_RestructuringCostAndReserveAxis
= us-gaap_RestructuringChargesMember