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Accounting Policies - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Line Items]          
Impairments $ 928,000 $ 832,000 $ 2,400,000 $ 4,808,000  
Level 3 [Member]          
Organization Consolidation And Presentation Of Financial Statements [Line Items]          
Assumed annual average rent increases for unobservable inputs     2.00%    
Level 3 [Member] | Measurement Input Expected Term [Member] | Maximum [Member]          
Organization Consolidation And Presentation Of Financial Statements [Line Items]          
Assumed holding periods for unobservable inputs     15 years    
Level 3 [Member] | Measurement Input Cap Rate [Member]          
Organization Consolidation And Presentation Of Financial Statements [Line Items]          
Real estate fair value, measurement input 8.00%   8.00%    
Level 3 [Member] | Measurement Input Discount Rate [Member]          
Organization Consolidation And Presentation Of Financial Statements [Line Items]          
Real estate fair value, measurement input 7.00%   7.00%    
Properties [Member]          
Organization Consolidation And Presentation Of Financial Statements [Line Items]          
Impairments     $ 610,000 821,000  
Estimated Sale Price Method [Member]          
Organization Consolidation And Presentation Of Financial Statements [Line Items]          
Impairments     2,400,000    
Estimated fair value $ 58,000   58,000    
Discounted Cash Flow Method [Member]          
Organization Consolidation And Presentation Of Financial Statements [Line Items]          
Impairments     0    
Accumulation of Asset Retirement Cost Method [Member]          
Organization Consolidation And Presentation Of Financial Statements [Line Items]          
Impairments     2,400,000    
Estimated fair value 2,342,000   2,342,000    
Notes and Mortgages Receivable [Member]          
Organization Consolidation And Presentation Of Financial Statements [Line Items]          
Impairments 0 0 0 0  
Direct Financing Leases Financing Receivable [Member]          
Organization Consolidation And Presentation Of Financial Statements [Line Items]          
Impairments 0 $ 0 0 $ 0  
Prepaid Expenses and Other Assets [Member]          
Organization Consolidation And Presentation Of Financial Statements [Line Items]          
Debt issuance costs $ 2,234,000   $ 2,234,000   $ 2,773,000