Annual report pursuant to Section 13 and 15(d)

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

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Our Portfolio - Analysis of Portfolio by Type of Obligor and Credit Quality (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]    
Equity method investments $ 522,615 $ 363,297
Receivables 993,000  
Receivables held-for-sale 19,081  
Real estate 362,000  
Investments 151,209 $ 58,058
Total 2,027,000  
Average Remaining Balance 11,000  
Renewable Energy Projects [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Equity method investments 502,000  
Commercial Investment Grade [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Receivables 464,000  
Receivables held-for-sale 3,000  
Real estate 341,000  
Investments 47,000  
Total 855,000  
Average Remaining Balance 9,000  
Commercial Non-Investment Grade [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Receivables 10,000  
Total 10,000  
Average Remaining Balance $ 5,000  
Credit Concentration Risk [Member] | Commercial Investment Grade [Member] | Debt and Real Estate Portfolio [Member]    
Financing Receivable, Recorded Investment [Line Items]    
% of Debt and real estate portfolio 57.00%  
Credit Concentration Risk [Member] | Commercial Non-Investment Grade [Member] | Debt and Real Estate Portfolio [Member]    
Financing Receivable, Recorded Investment [Line Items]    
% of Debt and real estate portfolio 1.00%  
Government [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Receivables $ 519,000  
Receivables held-for-sale 16,000  
Investments 104,000  
Total 639,000  
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%  
Subtotal, Debt and Real Estate [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Receivables $ 993,000  
Receivables held-for-sale 19,000  
Real estate 341,000  
Investments 151,000  
Total 1,504,000  
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%  
Equity Method Investments [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Real estate $ 21,000  
Total 523,000  
Average Remaining Balance 19,000  
Equity Method Investments [Member] | Renewable Energy Projects [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Equity method investments $ 502,000