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Discontinued Operations and Related Assets Held for Sale (Narratives) (Details) - USD ($) $ in Thousands |  | 1 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Jul. 03, 2017 | Dec. 31, 2016 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Nov. 30, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |
Proceeds from divestiture of businesses |  |  |  |  | $ 27,254 | $ 15,900 |
Gain (Loss) on Disposition of Business |  |  |  |  |  | (200) |
Fiber Materials Inc. [Member] |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |
Potential Earnings in Disposed Entity |  |  |  |  |  |  | $ 8,500
Discontinued Operations, Held-for-sale [Member] | Engineered Solutions [Member] |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |
Impairment |  |  | $ 0 | $ 5,300 | 5,300 | 120,000 |
Proceeds from divestiture of businesses |  |  |  |  | 27,254 | 15,889 |
Gain (Loss) on Disposition of Business |  |  | $ 0 | $ 0 | $ 6,091 | $ (198) |
Discontinued Operations, Held-for-sale [Member] | Fiber Materials Inc. [Member] |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |
Proceeds from divestiture of businesses |  | $ 15,900 |  |  |  |  |
Gain (Loss) on Disposition of Business |  | $ 200 |  |  |  |  |
Discontinued Operations, Held-for-sale [Member] | Advanced Energy Technologies |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |
Proceeds from divestiture of businesses | $ 28,500 |  |  |  |  |  |

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