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Business Segments - Additional Information (Details) $ in Millions |  | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | ---
Jul. 12, 2018 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) segment | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($)
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Number of business segments | segment |  |  |  | 2 |  |
Impairment of goodwill |  | $ 0.0 | $ 114.4 | $ 0.0 | $ 114.4 |
Goodwill |  | 536.4 |  | 536.4 |  | $ 531.4
Allegheny Technologies, Senior Notes, 9.375, Due 2019 [Member] |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Interest rate |  |  | 9.375% |  | 9.375% |
Addaero Manufacturing [Member] |  |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |  |
Payments to Acquire Businesses, Gross | $ 10.0 |  |  |  |  |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles |  | 2.0 |  | 2.0 |  |
Goodwill |  | $ 6.0 |  | $ 6.0 |  |

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