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Summary of Significant Accounting Policies - Additional Information (Detail)
 12 Months Ended
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Jan. 01, 2019
lease_option
Significant Accounting Policies [Line Items]        
Depreciation and amortization of deferred leasing costs $ 45,865,000 $ 40,490,000 $ 39,768,000  
Repairs and maintenance expense 11,100,000 12,500,000 11,900,000  
Impairment of real estate 0      
Rental revenue, lease related reserves 3,500,000      
Rental revenue, charge offs 1,700,000      
Deferred leasing costs, net 26,872,000 24,083,000    
Accumulated amortization deferred leasing cost (44,500,000) (41,600,000)    
Amortization expense deferred leasing cost 5,200,000 5,800,000 6,100,000  
Accrued penalties and interest 0 0 $ 0  
Tax basis of the company's real estate investments 1,550,000,000 $ 1,330,000,000    
Right-of-use asset 900,000      
Operating lease liability $ 900,000      
Operating Lease, Liability, Statement of Financial Position [Extensible List] us-gaap:LiabilitiesAbstract      
Corporate Headquarters        
Significant Accounting Policies [Line Items]        
Number of options to extend lease | lease_option       1
Lease renewal term       5 years
Minimum        
Significant Accounting Policies [Line Items]        
Operating lease term of contract       1 year
Minimum | Building        
Significant Accounting Policies [Line Items]        
Estimated useful life 35 years      
Maximum        
Significant Accounting Policies [Line Items]        
Operating lease term of contract       15 years
Maximum | Building        
Significant Accounting Policies [Line Items]        
Estimated useful life 50 years      
Maximum | Building Improvements        
Significant Accounting Policies [Line Items]        
Estimated useful life 20 years