Mortgage Notes Payable, Revolving Credit Facility, Interest Expense and Amortization of Deferred Debt Costs - Additional Information (Detail)								1 Months Ended	3 Months Ended	12 Months Ended
	Nov. 12, 2019 USD ($)	Jan. 10, 2019 USD ($)	Dec. 18, 2018 USD ($)	Oct. 03, 2018 USD ($)	Nov. 20, 2017 USD ($)	Aug. 14, 2017 USD ($)	Jan. 18, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2019 USD ($)	Oct. 01, 2021 USD ($)	Oct. 01, 2020 USD ($)	Nov. 21, 2019 USD ($)	Jun. 03, 2019 USD ($)	Jan. 04, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 01, 2017 USD ($)
Debt Instrument [Line Items]
Outstanding debt										$ 1,100,000,000
Fixed rate mortgages										821,503,000						$ 880,271,000
Variable rate debt										74,691,000						74,591,000
Principal amount										1,100,921,000						1,000,000,000.0
Debt outstanding with fixed-rate																910,200,000
Debt outstanding with variable-rate																122,000,000.0
Remaining loan balance										271,662,000
Unamortized deferred debt costs										9,733,000						10,300,000
Accumulated amortization										7,500,000						7,300,000
Capitalization of debt issuance cost										1,000,000.0						3,200,000	$ 2,600,000
Mortgage Notes Payable
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure										41,000,000.0						51,000,000.0
Fixed rate mortgage notes payable
Debt Instrument [Line Items]
Fixed rate mortgages										938,400,000						910,200,000
Fixed rate mortgage notes payable | 3.75% due December 2032 | Washington Square
Debt Instrument [Line Items]
Remaining loan balance					$ 28,100,000
Variable rate loans payable
Debt Instrument [Line Items]
Variable rate debt										162,500,000
New Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity										$ 400,000,000.0
Extension in line of credit facility period										1 year
Available borrowing capacity										$ 237,300,000
Outstanding line of credit										87,500,000
Letter of credit available borrowing capacity										185,000
New Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity										$ 325,000,000.0
Interest rate spread on LIBOR										1.35%
New Facility | Revolving Credit Facility | Term Facility
Debt Instrument [Line Items]
Interest rate spread on LIBOR										1.30%
New Facility | Term Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity										$ 75,000,000.0
Park Van Ness Loan
Debt Instrument [Line Items]
Fixed rate mortgages										68,100,000
Guarantor obligations, maximum exposure										6,700,000
Park Van Ness Loan | 4.88% due September 2032
Debt Instrument [Line Items]
Loan required periodic principal and interest payment								$ 413,460
Loan fixed interest rate																		4.88%
Amortization schedule								25 years
Balloon payment to be paid								$ 39,600,000
Kentlands Square I | Revolving Credit Facility | 4.41% mortgage maturing 2033
Debt Instrument [Line Items]
Remaining loan balance				$ 6,100,000
Kentlands Square II Bank Loan | Fixed rate mortgage notes payable
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure										8,500,000
Remaining loan balance										34,000,000.0
Burtonsville Fixed Rate Mortgage | 3.39% due January 2032
Debt Instrument [Line Items]
Loan amount							$ 40,000,000.0
Loan required periodic principal and interest payment							$ 197,900
Loan term							15 years
Loan fixed interest rate							3.39%
Amortization schedule							25 years
Balloon payment to be paid							$ 20,300,000
Waycroft Construction Loan | 4.67% Construction To Permanent Loan Due 2035
Debt Instrument [Line Items]
Loan amount						$ 157,000,000.0
Loan fixed interest rate						4.67%
Amortization schedule						25 years
Washington Square 2017 Mortgage | 3.75% due December 2032
Debt Instrument [Line Items]
Loan amount					60,000,000.0
Loan required periodic principal and interest payment					$ 308,500
Loan term					15 years
Loan fixed interest rate					3.75%
Amortization schedule					25 years
Balloon payment to be paid					$ 31,100,000
Broadlands Village
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure										3,900,000
Remaining loan balance										31,200,000
Broadlands Village | 4.41% mortgage maturing 2033
Debt Instrument [Line Items]
Remaining loan balance				15,200,000
Loan amount				32,000,000.0
Loan required periodic principal and interest payment				$ 176,200
Loan term				15 years
Loan fixed interest rate				4.41%
Amortization schedule				25 years
Balloon payment to be paid				$ 17,300,000
Avenel Business Park
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure										6,300,000
Remaining loan balance										26,300,000
The Glen Mortgage | 4.41% mortgage maturing 2033
Debt Instrument [Line Items]
Remaining loan balance				$ 7,300,000
The Glen Mortgage | 4.69% fixed rate loan maturing in 2034
Debt Instrument [Line Items]
Loan amount			$ 22,900,000
Loan required periodic principal and interest payment			$ 129,800
Loan term			15 years
Loan fixed interest rate			4.69%
Amortization schedule			25 years
Balloon payment to be paid			$ 12,500,000
Countryside
Debt Instrument [Line Items]
Remaining loan balance															$ 12,700,000
Olde Forte Village | Four Point Six Five Maturing January Twenty Thirty Four [Member]
Debt Instrument [Line Items]
Loan amount		$ 22,100,000
Loan required periodic principal and interest payment		$ 124,700
Loan term		15 years
Loan fixed interest rate		4.65%
Amortization schedule		25 years
Balloon payment to be paid		$ 12,100,000
Briggs Chancy MarketPlace
Debt Instrument [Line Items]
Remaining loan balance														$ 12,400,000
Shops at Monocacy | 4.69% fixed rate loan maturing in 2034
Debt Instrument [Line Items]
Balloon payment to be paid	$ 15,100,000
Shops at Monocacy | Four Point Six Five Maturing January Twenty Thirty Four [Member]
Debt Instrument [Line Items]
Loan amount	28,500,000
Loan required periodic principal and interest payment	$ 152,600
Loan term	15 years
Loan fixed interest rate	4.14%
Amortization schedule	25 years
Thruway
Debt Instrument [Line Items]
Remaining loan balance													$ 35,600,000
Debt Covenant
Debt Instrument [Line Items]
Carrying value of properties collateralized										$ 1,100,000,000						$ 1,100,000,000
Credit agreement leverage ratio										60.00%
Interest coverage ratio required minimum										2.0
Required fixed charge coverage ratio										1.4
Scenario, Forecast | Park Van Ness Loan
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure											$ 0	$ 3,300,000
Scenario, Forecast | Waycroft Construction Loan | 4.67% Construction To Permanent Loan Due 2035
Debt Instrument [Line Items]
Loan required periodic principal and interest payment									$ 887,900
Park Van Ness | Fixed rate mortgage notes payable | 4.88% due September 2032
Debt Instrument [Line Items]
Fixed rate mortgages																		$ 71,600,000
Minimum | New Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Interest rate spread on LIBOR										1.35%
Minimum | New Facility | Revolving Credit Facility | Term Facility
Debt Instrument [Line Items]
Interest rate spread on LIBOR										1.30%
Maximum | New Facility | Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Interest rate spread on LIBOR										1.95%
Maximum | New Facility | Revolving Credit Facility | Term Facility
Debt Instrument [Line Items]
Interest rate spread on LIBOR										1.90%