Quarterly report pursuant to Section 13 or 15(d)

Our Portfolio - Analysis of Portfolio by Type of Obligor and Credit Quality (Details)

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Our Portfolio - Analysis of Portfolio by Type of Obligor and Credit Quality (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]    
Equity method investments $ 434,463 $ 522,615
Receivables 1,081,000  
Receivables held-for-sale 16,000  
Real estate 384,000  
Investments 162,188 $ 151,209
Total 2,055,000  
Average remaining balance 11,000  
Renewable Energy Projects [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Equity method investments 412,000  
Commercial Investment Grade [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Receivables 457,000  
Receivables held-for-sale 0  
Real estate 362,000  
Investments 61,000  
Total 880,000  
Average remaining balance $ 9,000  
Commercial Investment Grade [Member] | Credit Concentration Risk [Member] | Debt and Real Estate Portfolio [Member]    
Financing Receivable, Recorded Investment [Line Items]    
% of Debt and real estate portfolio 55.00%  
Commercial Investment Grade [Member] | Renewable Energy Projects [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Equity method investments $ 0  
Commercial Non-Investment Grade [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Receivables 120,000  
Receivables held-for-sale 0  
Real estate 0  
Investments 0  
Total 120,000  
Average remaining balance $ 40,000  
Commercial Non-Investment Grade [Member] | Credit Concentration Risk [Member] | Debt and Real Estate Portfolio [Member]    
Financing Receivable, Recorded Investment [Line Items]    
% of Debt and real estate portfolio 7.00%  
Commercial Non-Investment Grade [Member] | Renewable Energy Projects [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Equity method investments $ 0  
Government [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Receivables 504,000  
Receivables held-for-sale 16,000  
Real estate 0  
Investments 101,000  
Total 621,000  
Average remaining balance $ 11,000  
Government [Member] | Credit Concentration Risk [Member] | Debt and Real Estate Portfolio [Member]    
Financing Receivable, Recorded Investment [Line Items]    
% of Debt and real estate portfolio 38.00%  
Government [Member] | Renewable Energy Projects [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Equity method investments $ 0  
Subtotal, Debt and Real Estate [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Receivables 1,081,000  
Receivables held-for-sale 16,000  
Real estate 362,000  
Investments 162,000  
Total 1,621,000  
Average remaining balance $ 10,000  
Subtotal, Debt and Real Estate [Member] | Credit Concentration Risk [Member] | Debt and Real Estate Portfolio [Member]    
Financing Receivable, Recorded Investment [Line Items]    
% of Debt and real estate portfolio 100.00%  
Subtotal, Debt and Real Estate [Member] | Renewable Energy Projects [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Equity method investments $ 0  
Equity Method Investments [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Receivables 0  
Receivables held-for-sale 0  
Real estate 22,000  
Investments 0  
Total 434,000  
Average remaining balance 16,000  
Equity Method Investments [Member] | Renewable Energy Projects [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Equity method investments $ 412,000