Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Sales Leaseback (Details)

v3.8.0.1
Summary of Significant Accounting Policies - Sales Leaseback (Details)
 12 Months Ended
Dec. 31, 2017
USD ($)
project
Dec. 31, 2016
USD ($)
project
Dec. 31, 2015
USD ($)
project
Sep. 30, 2016
USD ($)
Sale Leaseback Transaction [Line Items]        
Deferred Gain Recorded $ 6,173,000 $ 1,721,000    
Deferred Loss Recorded $ 2,172,000 $ 1,009,000    
Solar PV project        
Sale Leaseback Transaction [Line Items]        
Maximum combined funding amount       $ 100,000,000
Number of projects sold then leased back to separate investor | project 1      
Number of Solar PV Projects Sold | project 13 6 3  
Sale Price $ 51,204,000 $ 17,045,000 $ 12,506,000  
Deferred Gain Recorded 4,625,000 906,000 933,000  
Deferred Loss Recorded 1,204,000 145,000 924,000  
Capital Lease Asset/Liability Recorded $ 22,934,000 8,830,000 $ 6,810,000  
Initial Lease Term (years)     20 years  
Percentage of fair value threshold integral equipment 10.00%      
Net amortization expense $ (79,000) $ (24,000)    
Solar PV project | Minimum        
Sale Leaseback Transaction [Line Items]        
Initial Lease Term (years) 10 years 20 years    
Lease Payment $ 4,000 $ 2,000 $ 7,000  
Solar PV project | Maximum        
Sale Leaseback Transaction [Line Items]        
Initial Lease Term (years) 20 years 25 years    
Lease Payment $ 510,000 $ 397,000 $ 348,000