Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) ft² tenant store property shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($)
Significant Accounting Policies [Line Items]
Number of shopping centers | store	32
Percentage of ownership in operating partnership	74.41803%
Interest expense capitalized	$ 3,489,000	$ 2,527,000	$ 2,166,000
Depreciation and amortization of deferred leasing costs	40,197,000	38,755,000	37,698,000
Repairs and maintenance expense	11,600,000	11,800,000	11,600,000
Deferred leasing costs, net	27,255,000	25,983,000
Accumulated amortization deferred leasing cost	35,300,000	30,400,000
Amortization expense deferred leasing cost	5,500,000	5,600,000	5,600,000
Allowance for doubtful accounts receivables	405,000	1,958,000	1,263,000	$ 677,000
Accounts receivable, net representing rental income accrued	44,100,000	43,100,000
Allowance for doubtful accounts receivables representing rental income accrued	200,000	500,000
Real estate held-for-sale	3,400,000
Real estate, accumulated depreciation	488,166,000	458,279,000	425,370,000	$ 396,617,000
Real estate, costs to sell and liabilities	200,000
Unamortized deferred debt costs	6,906,000	7,500,000
Accumulated amortization	8,200,000	7,300,000
Accrued penalties and interest	0	0	$ 0
Tax basis of the company's real estate investments	$ 1,320,000,000	$ 1,260,000,000
Shares issued to directors (in shares) | shares	152,758	251,323	117,886
Shares issued to directors	$ 1,448,000	$ 1,348,000	$ 1,176,000
Director
Significant Accounting Policies [Line Items]
Shares credited to deferred fee accounts (in shares) | shares	183,818
Shares issued to directors (in shares) | shares	200	200	200
Shares issued to directors	$ 130,700	$ 150,100	$ 143,000
Property Held-for-sale
Significant Accounting Policies [Line Items]
Number of real estate properties | property	1
Net rentable area (in square feet) | ft²	197,100
Real estate, accumulated depreciation	$ 7,000,000
Minimum | Building
Significant Accounting Policies [Line Items]
Estimated useful life	35 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
Revenue
Significant Accounting Policies [Line Items]
Number of tenants | tenant	2
Revenue | Minimum
Significant Accounting Policies [Line Items]
Percentage of total revenue	2.50%
Revenue | Giant Food
Significant Accounting Policies [Line Items]
Percentage of total revenue	4.70%
Number of shopping centers | store	10
Revenue | Capital One
Significant Accounting Policies [Line Items]
Percentage of total revenue	2.80%
Number of shopping centers | store	18
Glebe Road Construction Loan
Significant Accounting Policies [Line Items]
Unamortized deferred debt costs	$ 1,800,000