Summary of Significant Accounting Policies - Narrative (Details) $ in Thousands		3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) tenant Property store shares	Sep. 30, 2018 USD ($) tenant Property store	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) tenant Property store shares	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Significant Accounting Policies [Line Items]
Number of tenants | tenant	1	1		1
Number of shopping centers | store	32	32		32
Percentage of ownership in operating partnership				74.10%
Allowance for doubtful accounts receivables	$ 600	$ 600		$ 600		$ 400
Accounts receivable, net representing rental income accrued	43,600	43,600		$ 43,600		44,100
Period expected for the sale of property to occur				1 year
Deferred debt costs	9,989	9,989		$ 9,989		6,900
Accumulated amortization, deferred debt costs	7,600	7,600		7,600		8,200
Deferred leasing costs, net	28,057	28,057		28,057		27,255
Accumulated amortization deferred leasing cost	$ 36,100	36,100		36,100		35,300
Amortization expense deferred leasing cost				4,300	$ 4,100
Interest capitalized		$ 1,716	$ 900	$ 4,301	2,495
Commercial development projects, availability for occupancy, maximum period after major construction activity				1 year
Depreciation expense				$ 29,700	30,300
Repairs and maintenance expense				$ 8,700	$ 8,500
Shares credited to directors' deferred fee accounts (in shares) | shares	113,040			6,686
Shares due to exercise of employee stock options and issuance of directors' deferred stock (in shares) | shares				32,740
Held-for-sale
Significant Accounting Policies [Line Items]
Number of real estate properties | Property	0	0		0
Minimum
Significant Accounting Policies [Line Items]
Allowance for doubtful accounts receivables representing rental income accrued	$ 200	$ 200		$ 200		200
Minimum | Building
Significant Accounting Policies [Line Items]
Estimated useful life (in years)				35 years
Maximum | Building
Significant Accounting Policies [Line Items]
Estimated useful life (in years)				50 years
Maximum | Building Improvements
Significant Accounting Policies [Line Items]
Estimated useful life (in years)				20 years
Giant Food
Significant Accounting Policies [Line Items]
Percentage of total revenue				4.80%
Number of shopping centers | store				10
Capital One
Significant Accounting Policies [Line Items]
Percentage of total revenue				2.70%
Number of shopping centers | store				18
Revenues
Significant Accounting Policies [Line Items]
Number of tenants | tenant	2	2		2
Percentage of total revenue				2.50%
Glebe Road Construction Loan
Significant Accounting Policies [Line Items]
Accumulated amortization, deferred debt costs						$ 1,800
Common Stock
Significant Accounting Policies [Line Items]
Shares due to exercise of employee stock options and issuance of directors' deferred stock (in shares) | shares				77,464