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Goodwill and Other Intangible Assets - Goodwill (Details) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2015 USD ($) | Dec. 31, 2016 USD ($) reporting_unit | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Jun. 12, 2014 USD ($)
Goodwill [Line Items] |  |  |  |  |
Goodwill | $ 651.4 | $ 641.9 | $ 651.4 |  |
Period increase (decrease) in goodwill |  | (9.5) |  |  |
Impairment of goodwill |  | 0.0 | (126.6) | $ 0.0 |
Accumulated goodwill impairment | 126.6 | 126.6 | $ 126.6 |  |
Hanard Machine, Inc. [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Goodwill |  |  |  |  | $ 8.4
High Performance Materials & Components [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Goodwill |  | 641.9 |  |  |
Impairment of goodwill |  | $ 0.0 |  | $ 0.0 |
Number of reporting units | reporting_unit |  | 3 |  |  |
High Performance Materials & Components [Member] | First Reporting Unit [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Goodwill |  | $ 470.8 |  |  |
Percentage of fair value in excess of carrying value |  | 14.00% |  |  |
High Performance Materials & Components [Member] | Second Reporting Unit [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Goodwill |  | $ 114.4 |  |  |
Percentage of fair value in excess of carrying value |  | 12.00% |  |  |
High Performance Materials & Components [Member] | Third Reporting Unit [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Goodwill |  | $ 56.7 |  |  |
Flat Rolled Products [Member] |  |  |  |  |
Goodwill [Line Items] |  |  |  |  |
Impairment of goodwill | $ (126.6) |  |  |  |

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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=64834752&loc=SL49117168-202975 + Details
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