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Summary of Significant Accounting Policies - Risks and Uncertainties and Use of Estimates (Details)
 12 Months Ended
Dec. 31, 2014
customer
Dec. 31, 2013
customer
Dec. 31, 2012
customer
Segment Reporting Information [Line Items]      
Number of employees 9,700dei_EntityNumberOfEmployees    
Percent of employees located outside of the U.S. 15.00%ati_EmployeesPercentNonU.S.    
Percent of employees covered by collective bargaining agreements 50.00%ati_EmployeesPercentCoveredbyCollectiveBargainingAgreements    
Sales Revenue [Member]      
Segment Reporting Information [Line Items]      
Number of customers 0ati_ConcentrationRiskNumberofCustomers
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueGoodsNetMember		 0ati_ConcentrationRiskNumberofCustomers
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueGoodsNetMember		 0ati_ConcentrationRiskNumberofCustomers
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueGoodsNetMember
Workforce Subject to Collective Bargaining Arrangements [Member] | Flat Rolled Products [Member]      
Segment Reporting Information [Line Items]      
Number of employees 2,300dei_EntityNumberOfEmployees
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_WorkforceSubjectToCollectiveBargainingArrangementsMember
/ us-gaap_StatementBusinessSegmentsAxis
= ati_FlatRolledProductsMember		    
Workforce Subject to Collective Bargaining Arrangements [Member] | High Performance Materials & Components [Member]      
Segment Reporting Information [Line Items]      
Number of employees 150dei_EntityNumberOfEmployees
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_WorkforceSubjectToCollectiveBargainingArrangementsMember
/ us-gaap_StatementBusinessSegmentsAxis
= ati_HighPerformanceMaterialsComponentsMember