Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2019 lease_option
Significant Accounting Policies [Line Items]
Depreciation and amortization of deferred leasing costs	$ 40,490,000	$ 39,768,000	$ 40,197,000
Repairs and maintenance expense	12,500,000	11,900,000	11,600,000
Allowance for doubtful accounts receivables	400,000	600,000
Accounts receivable, net representing rental income accrued	42,100,000	43,300,000
Allowance for doubtful accounts receivables representing rental income accrued	30,000	58,500
Deferred leasing costs, net	24,083,000	28,083,000
Accumulated amortization deferred leasing cost	(41,600,000)	(37,700,000)
Amortization expense deferred leasing cost	5,800,000	6,100,000	5,500,000
Accrued penalties and interest	0	0	$ 0
Tax basis of the company's real estate investments	1,330,000,000	$ 1,350,000,000
Right-of-use asset	1,600,000
Operating lease liability	$ 1,600,000
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
Corporate Headquarters
Significant Accounting Policies [Line Items]
Number of options to extend lease | lease_option				1
Lease renewal term				5 years