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Redeemable Noncontrolling Interest - Narrative (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Feb. 07, 2014
Noncontrolling Interest [Line Items]      
Redeemable noncontrolling interest $ 0us-gaap_TemporaryEquityIssuePeriodIncreaseOrDecrease    
Redeemable noncontrolling interest 12.1us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount 0us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount  
ATI Flowform [Member]      
Noncontrolling Interest [Line Items]      
Noncontrolling interest held by others     15.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
/ us-gaap_BusinessAcquisitionAxis
= ati_DynamicFlowformCorp.Member
Retained Earnings      
Noncontrolling Interest [Line Items]      
Redeemable noncontrolling interest $ (0.3)us-gaap_TemporaryEquityIssuePeriodIncreaseOrDecrease
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember