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Goodwill and Other Intangible Assets - Goodwill (Details) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2017 USD ($) reporting_unit | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Goodwill [Line Items] |  |  |  |  |  |
Goodwill | $ 531,400,000 |  |  | $ 531,400,000 | $ 641,900,000 |
Period increase (decrease) in goodwill |  |  |  | (110,500,000) |  |
Impairment of goodwill |  | $ (114,400,000) |  | (114,400,000) | 0 | $ (126,600,000)
Accumulated goodwill impairment | 241,000,000 |  |  | 241,000,000 | 126,600,000 |
High Performance Materials & Components [Member] |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |
Goodwill | 531,400,000 |  |  | 531,400,000 |  |
Impairment of goodwill | $ 0 |  |  | (114,400,000) | $ 0 |
Goodwill, Foreign Currency Translation Gain (Loss) |  |  |  | $ 3,900,000 |  |
Number of reporting units | reporting_unit |  |  |  | 2 |  |
High Performance Materials & Components [Member] | First Reporting Unit [Member] |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |
Percentage of fair value in excess of carrying value |  |  |  |  | 12.00% |
Flat Rolled Products [Member] |  |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |  |
Impairment of goodwill |  |  | $ (126,600,000) |  |  |

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