Accrued Liabilities and Other Current Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accrued Liabilities and Other Current Liabilities
Employee compensation expense	$ 2,389	$ 1,851
Project asset costs	6,930	3,317
Accrued legal services	140	141
Other accrued liabilities	433	547
Total accrued liabilities and other current liabilities	$ 9,892	$ 5,856