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Discontinued Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
Discontinued Operations (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets held for sale	$ 485		$ 485		$ 485
Accrued severance charges					$ 376
Loss from discontinued operations	(74)	$ (1,269)	(99)	$ (1,999)
Fuel Conversion Segment
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Expected severance costs	581		581
Fuel Conversion Segment \| Period Two
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cost remaining	311		311
Fuel Conversion Segment \| Period One
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Severance costs	94		203
Fuel Conversion Segment \| Period Three
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cost remaining	65		65
Fuel Conversion Segment \| Accrued Liabilities
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Accrued severance charges	188		$ 188
Held-for-sale \| Patent
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Intangible asset impairment	$ 56