Accrued Liabilities and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued Liabilities and Other Current Liabilities
Employee compensation expense	$ 1,851	$ 1,535
Project asset costs	3,317	1,810
Accrued legal services	141	91
Other accrued liabilities	547	331
Total accrued liabilities and other current liabilities	5,856	3,767
Non-current compensation liabilities related to engineering staff	700	1,100
Non-current compensation liabilities related to executive staff	$ 1,300	$ 1,600