Quarterly report pursuant to Section 13 or 15(d)

Our Portfolio - Equity Method Investments (Details)

v3.8.0.1
Our Portfolio - Equity Method Investments (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]    
Carrying Value $ 490,297 $ 522,615
Vento I, LLC [Member]    
Schedule of Equity Method Investments [Line Items]    
Carrying Value 115,000  
Northern Frontier, LLC [Member]    
Schedule of Equity Method Investments [Line Items]    
Carrying Value 113,000  
Buckeye Wind Energy Class B Holdings, LLC [Member]    
Schedule of Equity Method Investments [Line Items]    
Carrying Value 65,000  
Invenergy Gunsight Mountain Holdings, LLC [Member]    
Schedule of Equity Method Investments [Line Items]    
Carrying Value 37,000  
MM Solar Holdings, LLC [Member]    
Schedule of Equity Method Investments [Line Items]    
Carrying Value 27,000  
Helix Fund, LLC [Member]    
Schedule of Equity Method Investments [Line Items]    
Carrying Value 24,000  
Other Transactions [Member]    
Schedule of Equity Method Investments [Line Items]    
Carrying Value $ 109,000