Quarterly report pursuant to Section 13 or 15(d)

Our Portfolio - Receivables and Investments, Additional Information (Details)

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Our Portfolio - Receivables and Investments, Additional Information (Details) - USD ($)
 1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Nov. 02, 2018
Jan. 31, 2018
Sep. 30, 2018
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]            
Proceeds from sales of equity method investments       $ 12,433,000 $ 6,044,000  
Financing receivables on non accrual status     $ 0 0   $ 0
Provision for credit losses       0   0
Loan modifications that qualify as troubled debt restructurings       0   $ 0
Land [Member]            
Financing Receivable, Recorded Investment, Past Due [Line Items]            
Proceeds from lease payments   $ 1,600,000        
American Wind Capital Company, LLC [Member]            
Financing Receivable, Recorded Investment, Past Due [Line Items]            
Commercial receivables, carrying value that became past due     8,000,000 8,000,000    
Subsequent Event [Member]            
Financing Receivable, Recorded Investment, Past Due [Line Items]            
Guarantor Obligations, Maximum Exposure, Undiscounted $ 300          
Commercial Non-Investment Grade [Member] | Residential Solar Projects [Member]            
Financing Receivable, Recorded Investment, Past Due [Line Items]            
Proceeds from sales of equity method investments     12,000,000      
Gain realized from sale of investment     1,000,000      
SunStrong [Member] | Subsequent Event [Member]            
Financing Receivable, Recorded Investment, Past Due [Line Items]            
Future repayments of senior debt $ 308,000,000          
SunStrong [Member] | Subsequent Event [Member] | SunPower Corporation [Member]            
Financing Receivable, Recorded Investment, Past Due [Line Items]            
Voting interest to be acquired (as a percent) 49.00%          
Purchase price $ 10,000,000          
SunStrong [Member] | Subsequent Event [Member] | Mezzanine Loan [Member]            
Financing Receivable, Recorded Investment, Past Due [Line Items]            
Interest rate on loans 11.75%          
Maximum lending limit $ 32,000,000          
SunStrong [Member] | Residential Solar Loan [Member]            
Financing Receivable, Recorded Investment, Past Due [Line Items]            
Loans receivable     $ 110,000,000 $ 110,000,000    
Interest rate on loans     12.00% 12.00%    
SunStrong [Member] | Residential Solar Loan [Member] | Senior Debt Obligations [Member]            
Financing Receivable, Recorded Investment, Past Due [Line Items]            
Loans receivable     $ 308,000,000 $ 308,000,000