Notes Payable, Revolving Credit Facility, Interest and Amortization of Deferred Debt Costs - Narrative (Details) - USD ($)					9 Months Ended
	Jul. 24, 2020	Jul. 14, 2020	Mar. 03, 2020	Feb. 10, 2020	Sep. 30, 2020	Sep. 30, 2019	Oct. 01, 2021	Oct. 01, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Principal amount					$ 1,185,518,000				$ 1,100,000,000
Debt outstanding with fixed-rate					986,000,000.0				938,400,000
Debt outstanding with variable-rate					199,500,000				162,500,000
Carrying value of properties collateralizing mortgage notes					1,200,000,000				1,100,000,000
Outstanding line of credit					123,778,000				86,371,000
Repayments of secured debt					37,431,000	$ 55,992,000
Proceeds from revolving credit facility					90,000,000	$ 47,000,000
Principal amount					1,175,783,000
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid					909,141,000
Notes payable					835,613,000				821,503,000
Unamortized deferred debt costs					9,735,000				9,700,000
Debt issuance costs amortization					8,300,000				7,500,000
Unsecured Revolving Credit Facility
Debt Instrument [Line Items]
Line of credit facility, remaining borrowing capacity					200,300,000
Outstanding line of credit					124,500,000				$ 87,500,000
Letter of Credit
Debt Instrument [Line Items]
Outstanding line of credit					185,000
New Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					$ 400,000,000.0
New Facility | Revolving Credit Facility | LIBOR
Debt Instrument [Line Items]
Interest rate spread on LIBOR					1.40%
New Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					$ 325,000,000.0
Proceeds from revolving credit facility					$ 71,000,000.0
New Facility | Term Facility | LIBOR
Debt Instrument [Line Items]
Interest rate spread on LIBOR					1.35%
New Facility | Term Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					$ 75,000,000.0
Boca Valley Plaza Loan
Debt Instrument [Line Items]
Repayments of secured debt				$ 9,200,000
Palm Springs Center Loan
Debt Instrument [Line Items]
Repayments of secured debt			$ 7,100,000
Park Van Ness Loan
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure					6,700,000
Notes payable					66,800,000
Park Van Ness Loan | Subsequent Event
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure							$ 0	$ 3,300,000
Broadlands Village Mortgage
Debt Instrument [Line Items]
Principal amount					30,700,000
Guarantor obligations, maximum exposure					3,800,000
Avenel Business Park
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure					6,300,000
Avenel Business Park | Mortgages
Debt Instrument [Line Items]
Principal amount					25,500,000
The Waycroft Mortgage
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure					23,600,000
The Waycroft Mortgage | Mortgages
Debt Instrument [Line Items]
Principal amount					143,100,000
Ashbrook Marketplace Mortgage | Mortgages
Debt Instrument [Line Items]
Long-term debt, term		15 years
Principal amount		$ 22,100,000			22,100,000
Debt instrument, interest rate, stated percentage		3.80%
Debt Instrument, Periodic Payment		$ 114,226
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid		$ 11,500,000
Kentlands Place Mortgage | Mortgages
Debt Instrument [Line Items]
Long-term debt, term	15 years
Principal amount	$ 30,000,000.0				$ 29,900,000
Debt instrument, interest rate, stated percentage	3.43%
Debt Instrument, Periodic Payment	$ 149,064
Debt Instrument, Periodic Payment Terms, Balloon Payment to be Paid	$ 15,300,000