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Discontinued Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
Discontinued Operations (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets held for sale	$ 1,839		$ 1,839		$ 2,058
Loss from discontinued operations	(239)	$ (630)	(2,238)	$ (2,144)
Research and development	216	$ 672	780	$ 1,438
Fuel Conversion Segment
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Expected severance costs	580		580
Fuel Conversion Segment \| Period One
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Expected severance costs	200		200
Severance costs	170
Cost remaining	30		30
Fuel Conversion Segment \| Period Two
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cost remaining	320		320
Fuel Conversion Segment \| Period Three
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cost remaining	60		60
Fuel Conversion Segment \| Unclassified Indefinite-lived Intangible Assets
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets held for sale	1,354		1,354
Fuel Conversion Segment \| Equipment
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets held for sale	485		485
Fuel Conversion Segment \| Accrued Liabilities
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Accrued severance charges	243		243
Fuel Conversion Segment \| Accrued Liabilities \| Employee Severance
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Research and development	$ 396		$ 396