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
3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]    
Equity method investments $ 490,297 $ 522,615
Receivables 985,000  
Receivables held-for-sale 16,000  
Investments 151,920 $ 151,209
Real estate 361,000  
Total 1,983,000  
Average remaining balance 11,000  
Renewable Energy Projects [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Equity method investments 469,000  
Commercial Investment Grade [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Receivables 463,000  
Receivables held-for-sale 0  
Investments 49,000  
Real estate 340,000  
Total 852,000  
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 57.00%  
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 8,000  
Receivables held-for-sale 0  
Investments 0  
Real estate 0  
Total 8,000  
Average remaining balance $ 4,000  
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 1.00%  
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 514,000  
Receivables held-for-sale 16,000  
Investments 103,000  
Real estate 0  
Total 633,000  
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 42.00%  
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 985,000  
Receivables held-for-sale 16,000  
Investments 152,000  
Real estate 340,000  
Total 1,493,000  
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  
Investments 0  
Real estate 21,000  
Total 490,000  
Average remaining balance 18,000  
Equity Method Investments [Member] | Renewable Energy Projects [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Equity method investments $ 469,000