Accrued Liabilities and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accrued Liabilities and Other Current Liabilities
Employee compensation expense	$ 3,062	$ 1,851
Project asset costs	6,056	3,317
Invitation to bid contract costs	4,418
Accrued legal services	180	141
Accrued equity issuance costs	301
Other accrued liabilities	539	547
Total accrued liabilities and other current liabilities	$ 14,556	$ 5,856