Quarterly report pursuant to Section 13 or 15(d)

Our Portfolio - Additional Information (Detail)

v3.8.0.1
Our Portfolio - Additional Information (Detail)
 3 Months Ended 9 Months Ended 12 Months Ended
Mar. 31, 2017
USD ($)
a
Project
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
Jun. 30, 2017
USD ($)
Financing Receivable, Recorded Investment [Line Items]          
Financing receivables, investments, real estate and equity method investments   $ 2,047,000,000      
Financing receivables on non accrual status   0   $ 0  
Loan modifications that qualify as trouble debt restructurings   0   0  
Provision for credit losses   0   0  
Land purchased | a 4,000        
Number of solar projects | Project 20        
Portfolio cost $ 145,000,000 142,952,000 $ 10,964,000    
Deferred funding obligations   225,817,000   170,892,000  
Escrow deposits   165,000,000      
Investment securities pledged as collateral   29,000,000   41,000,000  
Investment Contracts [Member]          
Financing Receivable, Recorded Investment [Line Items]          
Investment in project   10,000,000      
Financing Receivables and Investments [Member]          
Financing Receivable, Recorded Investment [Line Items]          
Deferred funding obligations   $ 20,000,000   $ 34,000,000  
American Wind Capital Company, LLC ("AWCC") [Member]          
Financing Receivable, Recorded Investment [Line Items]          
Financing receivables, carrying value that became past due         $ 10,000,000
Real Estate Portfolio [Member]          
Financing Receivable, Recorded Investment [Line Items]          
Real estate portfolio acquired value through equity interest in joint venture $ 21,000,000        
Joint Venture [Member]          
Financing Receivable, Recorded Investment [Line Items]          
Land purchased | a 1,100        
Lease Intangibles [Member]          
Financing Receivable, Recorded Investment [Line Items]          
Portfolio cost $ 56,000,000        
Minimum [Member]          
Financing Receivable, Recorded Investment [Line Items]          
Long-term triple net leases expiration dates range   2033      
Long-term triple net leases extended expiration dates range   2047      
Maximum [Member]          
Financing Receivable, Recorded Investment [Line Items]          
Long-term triple net leases expiration dates range   2057      
Long-term triple net leases extended expiration dates range   2080