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Financial Information for Subsidiary and Guarantor Parent Financial Information for Subsidiary and Guarantor Parent (Narrative) (Details) (Allegheny Ludlum 6.95% debentures due 2025, USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Allegheny Ludlum 6.95% debentures due 2025
   
Debt Instrument [Line Items]    
Long-term Debt, Gross $ 150.0us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= ati_AlleghenyLudlumDebenturesDue2025Member
$ 150.0us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= ati_AlleghenyLudlumDebenturesDue2025Member
Interest rate 6.95%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ati_AlleghenyLudlumDebenturesDue2025Member