Annual report pursuant to Section 13 and 15(d)

Borrowings - Metlife Facility (Narrative) (Details)

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Borrowings - Metlife Facility (Narrative) (Details)
 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 15, 2017
USD ($)
Oct. 05, 2016
USD ($)
Oct. 04, 2016
USD ($)
May 09, 2014
USD ($)
Debt Instrument [Line Items]          
Debt instrument, fee amount $ 276,000        
Professional fees 713,000        
2015 Metlife Term Note          
Debt Instrument [Line Items]          
Debt instrument, face amount         $ 100,000,000
2015 Metlife Line of Credit          
Debt Instrument [Line Items]          
Debt instrument, face amount         $ 25,000,000
MetLife Credit Facility          
Debt Instrument [Line Items]          
Debt instrument, face amount 200,000,000        
Line of credit facility, maximum borrowing capacity $ 75,000,000   $ 200,000,000 $ 125,000,000  
2016 Metlife Term Note          
Debt Instrument [Line Items]          
Line of credit facility, current borrowing capacity   $ 100,000,000 50,000,000    
2016 MetLife Line of Credit          
Debt Instrument [Line Items]          
Line of credit facility, current borrowing capacity   $ 50,000,000 $ 25,000,000    
New MetLife Credit Facility | Maximum          
Debt Instrument [Line Items]          
Line of credit facility, unused capacity, commitment fee percentage 0.20%        
New MetLife Credit Facility | Minimum          
Debt Instrument [Line Items]          
Line of credit facility, unused capacity, commitment fee percentage 0.10%        
2017 Amendment of MetLife Facility          
Debt Instrument [Line Items]          
Debt, long-term and short-term, combined amount, maximum borrowing capacity $ 275,000,000        
Debt instrument, fee amount 213,000        
2016 Amendment of MetLife Facility          
Debt Instrument [Line Items]          
Debt instrument, fee amount 225,000        
Prior MetLife Line Of Credit Facility          
Debt Instrument [Line Items]          
Deferred financing costs related to mortgage notes and bonds payable $ 299,000        
Mortgage notes payable | MetLife Credit Facility          
Debt Instrument [Line Items]          
Debt Instrument, loans to value ratios percentage 60.00%        
Mortgage notes payable | New MetLife Credit Facility          
Debt Instrument [Line Items]          
Debt instrument, face amount $ 200,000,000        
Metlife          
Debt Instrument [Line Items]          
Debt instrument, fee amount $ 658,000        
Number of collateralized properties 33        
Book value of collateralized asset $ 187,400,000