Notes Payable, Revolving Credit Facility, Interest and Amortization of Deferred Debt Costs - Narrative (Details) - USD ($)			3 Months Ended
	Mar. 03, 2020	Feb. 10, 2020	Mar. 31, 2020	Mar. 31, 2019	Oct. 01, 2021	Oct. 01, 2020	Dec. 31, 2019	Sep. 30, 2019
Debt Instrument [Line Items]
Principal amount			$ 1,128,432,000				$ 1,100,000,000
Debt outstanding with fixed-rate			928,900,000				938,400,000
Debt outstanding with variable-rate			199,500,000				162,500,000
Carrying value of properties collateralizing mortgage notes			1,100,000,000				1,100,000,000
Repayments of secured debt			23,352,000	$ 29,030,000
Notes payable			798,343,000				821,503,000
Principal amount			1,119,076,000
Outstanding line of credit			123,507,000				86,371,000
Unamortized deferred debt costs			9,356,000				9,700,000
Debt issuance costs amortization			7,500,000				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]
Line of credit facility, maximum borrowing capacity			185,000
New Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity			400,000,000.0
New Facility | Term Loan | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity			$ 75,000,000.0
New Facility | Term Loan | LIBOR
Debt Instrument [Line Items]
Interest rate spread on LIBOR (percentage)			1.30%
New Facility | Revolving Line | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity			$ 325,000,000.0
New Facility | Revolving Line | LIBOR
Debt Instrument [Line Items]
Interest rate spread on LIBOR (percentage)			1.35%
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								$ 67,700,000
Broadlands Village Mortgage
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure								3,900,000
Principal amount								31,000,000.0
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								$ 26,000,000.0
Forecast | Park Van Ness Loan
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure					$ 0	$ 3,300,000