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Redeemable Noncontrolling Interest Redeemable Noncontrolling Interest (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Feb. 07, 2014
Redeemable Noncontrolling Interest [Line Items]      
Redeemable noncontrolling interest $ 0us-gaap_TemporaryEquityIssuePeriodIncreaseOrDecrease    
Redeemable noncontrolling interest 12.1us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount 12.1us-gaap_RedeemableNoncontrollingInterestEquityCarryingAmount  
Dynamic Flowform [Member]      
Redeemable Noncontrolling Interest [Line Items]      
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners     15.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
/ us-gaap_BusinessAcquisitionAxis
= ati_DynamicFlowformMember
[RetainedEarningsMember]      
Redeemable Noncontrolling Interest [Line Items]      
Redeemable noncontrolling interest $ (0.1)us-gaap_TemporaryEquityIssuePeriodIncreaseOrDecrease
/ us-gaap_StatementEquityComponentsAxis
= us-gaap_RetainedEarningsMember