Annual report pursuant to Section 13 and 15(d)

Our Portfolio - Additional Information (Detail)

v3.8.0.1
Our Portfolio - Additional Information (Detail)
 1 Months Ended 3 Months Ended 12 Months Ended
Jan. 31, 2018
USD ($)
Mar. 31, 2017
USD ($)
a
Project
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Jun. 30, 2017
USD ($)
Financing Receivable, Recorded Investment [Line Items]            
Equity method investments, receivables, real estate and investments     $ 2,027,000,000      
Equity method investments     522,615,000 $ 363,297,000    
Payment received     98,482,000 116,432,000 $ 70,093,000  
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   $ 145,000,000 170,982,000 17,693,000 $ 42,913,000  
Deferred funding obligations     153,308,000 170,892,000    
Escrow deposits     90,000,000      
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      
Investment Contracts [Member] | Wind Projects [Member]            
Financing Receivable, Recorded Investment [Line Items]            
Equity method investments     25,000,000      
Financing Receivables and Investments [Member]            
Financing Receivable, Recorded Investment [Line Items]            
Deferred funding obligations     20,000,000 34,000,000    
Imputed interest related to certain of deferred funding obligations     $ 2,000,000 $ 1,000,000    
American Wind Capital Company, LLC ("AWCC") [Member]            
Financing Receivable, Recorded Investment [Line Items]            
Commercial receivables, carrying value that became past due           $ 10,000,000
American Wind Capital Company, LLC ("AWCC") [Member] | Subsequent Event [Member]            
Financing Receivable, Recorded Investment [Line Items]            
Payment received $ 1,600,000          
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