Quarterly report pursuant to Section 13 or 15(d)

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

v3.8.0.1
Our Portfolio - Analysis of Portfolio by Type of Obligor and Credit Quality (Detail) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]    
Financing receivables $ 1,062,000  
Investments 130,540 $ 58,058
Investments 130,540 58,058
Real estate 335,000  
Equity method investments 540,150 $ 363,297
Total 2,047,000  
Average remaining balance 12,000  
Renewable Energy Projects [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Equity method investments 519,000  
Commercial Investment Grade [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Financing receivables 478,000  
Investments 26,000  
Investments 26,000  
Real estate 314,000  
Total 818,000  
Average remaining balance 9,000  
Commercial Non-Investment Grade [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Financing 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 54.00%  
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]    
Financing receivables $ 574,000  
Investments 105,000  
Investments 105,000  
Total 679,000  
Average remaining balance $ 12,000  
Government [Member] | Credit Concentration Risk [Member] | Debt and Real Estate Portfolio [Member]    
Financing Receivable, Recorded Investment [Line Items]    
% of Debt and real estate portfolio 45.00%  
Subtotal, Debt and Real Estate [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Financing receivables $ 1,062,000  
Investments 131,000  
Investments 131,000  
Real estate 314,000  
Total 1,507,000  
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 540,000  
Average remaining balance 20,000  
Equity Method Investments [Member] | Renewable Energy Projects [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Equity method investments $ 519,000