Annual report pursuant to Section 13 and 15(d)

Mortgage Notes Payable, Revolving Credit Facility, and Term Loan Facility - Additional Information (Detail)

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Mortgage Notes Payable, Revolving Credit Facility, and Term Loan Facility - Additional Information (Detail)
12 Months Ended
Oct. 27, 2017
Jun. 22, 2017
USD ($)
Oct. 05, 2015
USD ($)
Dec. 31, 2017
USD ($)
property
mortgage
Jul. 31, 2017
Jul. 07, 2017
Dec. 31, 2016
USD ($)
property
Aug. 31, 2013
USD ($)
Debt Instrument [Line Items]                
Number of properties collateralized in mortgage notes payable | property       96        
Net book value of collateralized mortgage properties       $ 5,900,000        
Number of properties acquired in connection with fixed rate | property       5        
Number of properties acquired | property       7     3  
Derivative asset       $ 504,000     $ 101,000  
Fair value of mortgage notes payable outstanding       455,100,000        
Long-term debt       542,627,000     509,395,000  
Line of credit facility, maximum borrowing capacity       75,000,000.0       $ 25,000,000.0
Debt instrument, fee 0.9              
Borrowings under revolving credit facility, net       $ 20,715,000     39,225,000  
Line of credit at an interest rate (as percent)       3.28%        
Letters of credit, outstanding       $ 1,000,000        
Weighted average interest rate on debt outstanding (as percent)       4.42%        
Mortgage Notes Payable                
Debt Instrument [Line Items]                
Number of mortgage notes payable | mortgage       45        
Number of properties collateralized in mortgage notes payable | property       67        
Net book value of collateralized mortgage properties       $ 654,900,000        
Collateral       $ 11,700,000        
Collateral (as percent)       2.60%        
Number of mortgages repaid | mortgage       4        
Number of properties collateralized | property       10        
Number of long-term mortgages issued | property       4        
Number of properties to issue collateralized mortgage notes payable | property       7        
Long-term debt       $ 447,380,000     $ 445,278,000  
Carrying value of debt       $ 452,491,000        
Weighted average interest rate on debt outstanding (as percent)       4.56%        
New Fixed Rate Mortgage Notes Payable                
Debt Instrument [Line Items]                
Noncash or part noncash acquisition, debt assumed       $ 54,900,000        
Number of properties acquired | property       7        
Weighted average interest rate on debt outstanding (as percent)       3.78%        
New Variable Rate Mortgage Notes Payable                
Debt Instrument [Line Items]                
Debt instrument, interest rate, stated percentage       2.75%        
Additional Advance On New Variable Rate Mortgage Notes Payable [Member]                
Debt Instrument [Line Items]                
Debt instrument, interest rate, stated percentage       2.50%        
Minimum | Philadelphia, PA | New Fixed Rate Mortgage Notes Payable                
Debt Instrument [Line Items]                
Debt instrument, interest rate, stated percentage           3.75%    
Maximum | Maitland, Florida | New Fixed Rate Mortgage Notes Payable                
Debt Instrument [Line Items]                
Debt instrument, interest rate, stated percentage         3.89%      
Interest Rate Swap                
Debt Instrument [Line Items]                
Number of properties acquired in connection with interest rate swap | property       1        
Interest rate swap assumed   1.80%            
Interest Rate Swap | New Fixed Rate Mortgage Notes Payable                
Debt Instrument [Line Items]                
Debt instrument, interest rate, stated percentage       3.545%        
Interest Rate Swap | Conshohocken, Pennsylvania                
Debt Instrument [Line Items]                
Noncash or part noncash acquisition, debt assumed   $ 11,200,000            
Derivative asset   $ 40,000   $ 300,000        
London Interbank Offered Rate (LIBOR)                
Debt Instrument [Line Items]                
Libor rate (as percent) 2.75%     1.56%        
Five Year Term Loan Facility                
Debt Instrument [Line Items]                
Debt instrument term 5 years              
Revolving Credit Facility                
Debt Instrument [Line Items]                
Debt instrument term 4 years              
Line of Credit                
Debt Instrument [Line Items]                
Line of credit facility, maximum borrowing capacity     $ 85,000,000.0          
Borrowings under revolving credit facility, net       $ 96,400,000        
Line of credit facility, maximum additional amount drawn       $ 43,200,000        
Line of Credit | Five Year Term Loan Facility                
Debt Instrument [Line Items]                
Line of credit facility, maximum borrowing capacity     $ 25,000,000.0          
Debt instrument term     5 years          
Letter of Credit                
Debt Instrument [Line Items]                
Weighted average interest rate on debt outstanding (as percent)       1.75%