Quarterly report pursuant to Section 13 or 15(d)

Our Portfolio - Financing Receivables, Investments, Real Estate and Equity Method Investments - Additional Information (Detail)

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Our Portfolio - Financing Receivables, Investments, Real Estate and Equity Method Investments - Additional Information (Detail) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]          
Financing receivables, investments, real estate and equity method investments $ 1,116,000,000   $ 1,116,000,000    
Deferred funding obligations 95,700,000   95,700,000   $ 88,288,000
Restricted cash 400,000   400,000   3,000,000
Charged off of receivable against allowance     1,200,000    
Allowance for credit losses 0   0   1,000,000
Other financing receivables on non accrual status 0   0   $ 0
Provision for credit losses 0     $ 0  
Loan modifications that qualify as trouble debt restructurings $ 0 $ 0      
Energy Source LLC [Member]          
Financing Receivable, Recorded Investment [Line Items]          
Recovered of loan outstanding balance     $ 800,000    
Credit Concentration Risk [Member] | Debt and Real Estate Portfolio [Member]          
Financing Receivable, Recorded Investment [Line Items]          
% of Debt and Real Estate Portfolio     99.00%    
Minimum [Member]          
Financing Receivable, Recorded Investment [Line Items]          
Long term land lease agreements expiration dates range     2033    
Long term land lease agreements extended expiration dates range     2047    
Maximum [Member]          
Financing Receivable, Recorded Investment [Line Items]          
Long term land lease agreements expiration dates range     2044    
Long term land lease agreements extended expiration dates range     2061