Notes Payable, Revolving Credit Facility, Interest and Amortization of Deferred Debt Costs - Narrative (Details) - USD ($)			6 Months Ended
	Mar. 03, 2020	Feb. 10, 2020	Jun. 30, 2020	Jun. 30, 2019	Oct. 01, 2021	Oct. 01, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Principal amount			$ 1,183,604,000				$ 1,100,000,000
Debt outstanding with fixed-rate			934,100,000				938,400,000
Debt outstanding with variable-rate			249,500,000				162,500,000
Carrying value of properties collateralizing mortgage notes			1,100,000,000				1,100,000,000
Outstanding line of credit			173,642,000				86,371,000
Repayments of secured debt			30,293,000	$ 48,715,000
Proceeds from revolving credit facility			90,000,000	$ 36,000,000
Notes payable			791,534,000				821,503,000
Principal amount			1,174,567,000
Unamortized deferred debt costs			9,037,000				9,700,000
Debt issuance costs amortization			7,900,000				7,500,000
Unsecured Revolving Credit Facility
Debt Instrument [Line Items]
Line of credit facility, remaining borrowing capacity			150,300,000
Outstanding line of credit			174,500,000				$ 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 Line | LIBOR
Debt Instrument [Line Items]
Interest rate spread on LIBOR (percentage)			1.40%
New Facility | Revolving Line | 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 Loan | LIBOR
Debt Instrument [Line Items]
Interest rate spread on LIBOR (percentage)			1.35%
New Facility | Term Loan | 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			67,300,000
Park Van Ness Loan | Forecast
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure					$ 0	$ 3,300,000
Broadlands Village Mortgage
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure			3,900,000
Principal amount			30,800,000
Avenel Business Park
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure			6,300,000
Avenel Business Park | Fixed Rate Mortgage Notes Payable
Debt Instrument [Line Items]
Principal amount			25,800,000
The Waycroft Mortgage
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure			23,600,000
The Waycroft Mortgage | Fixed Rate Mortgage Notes Payable
Debt Instrument [Line Items]
Principal amount			$ 136,200,000