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Fair Value Of Financial Instruments - Narrative (Details) | Dec. 31, 2017 | Dec. 31, 2016 | Feb. 15, 2016 | [1] | Dec. 31, 2013
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Allegheny Technologies $350 million 9.375% Senior Notes due 2019 [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Debt instrument interest rate stated percentage |  | 9.375% |  |
Allegheny Technologies $500 million 5.95% Senior Notes due 2021 [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Debt instrument interest rate stated percentage | 5.95% | 5.95% |  |
Allegheny Technologies $500 million 5.95% Senior Notes due 2021 [Member] | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Debt instrument interest rate stated percentage | 5.95% |  |  |
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Debt instrument interest rate stated percentage | 4.75% | 4.75% |  |
Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Debt instrument interest rate stated percentage |  |  | 7.875% | 5.875%
Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member] | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Debt instrument interest rate stated percentage |  |  |  | 5.875%
Allegheny Ludlum 6.95% Debentures due 2025 [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Debt instrument interest rate stated percentage | 6.95% | 6.95% |  |
Allegheny Ludlum 6.95% Debentures due 2025 [Member] | Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Debt instrument interest rate stated percentage | 6.95% |  |  |
Convertible Debt [Member] | Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Debt instrument interest rate stated percentage | 4.75% |  |  |
[1] | Bearing interest at 7.875% effective February 15, 2016.

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