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Mortgage Notes Payable, Revolving Credit Facility, Interest Expense and Amortization of Deferred Debt Costs - Additional Information (Detail) - USD ($)						1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 18, 2018	Oct. 03, 2018	Nov. 20, 2017	Aug. 14, 2017	Jan. 18, 2017	Sep. 30, 2017	Dec. 31, 2021	Dec. 31, 2018	Oct. 01, 2021	Oct. 01, 2020	Oct. 01, 2019	Dec. 31, 2017	Sep. 01, 2017	Dec. 31, 2016	Nov. 30, 2016
Debt Instrument [Line Items]
Outstanding debt								$ 1,000,000,000
Fixed rate mortgages								880,271,000				$ 897,888,000
Variable rate debt								74,591,000				0
Principal amount								1,032,189,000				965,500,000
Debt outstanding with fixed-rate												890,400,000
Debt outstanding with variable-rate												75,100,000
Remaining loan balance								251,361,000
Capitalization of debt issuance cost								3,200,000				2,600,000		$ 100,000
Mortgage Notes Payable
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure								51,000,000				51,000,000
Fixed rate mortgage notes payable
Debt Instrument [Line Items]
Fixed rate mortgages								910,189,000				890,393,000
Fixed rate mortgage notes payable \| Washington Square
Debt Instrument [Line Items]
Remaining loan balance			$ 28,100,000
Fixed rate mortgage notes payable \| 3.51% due June 2028
Debt Instrument [Line Items]
Fixed rate mortgages								38,120,000				39,968,000
Fixed rate mortgage notes payable \| 3.51% due June 2028 \| Beacon Center
Debt Instrument [Line Items]
Loan amount															$ 11,250,000.00
Loan required periodic principal and interest payment								268,500
Fixed rate mortgage notes payable \| 3.39% due January 2032
Debt Instrument [Line Items]
Fixed rate mortgages								38,076,000				39,140,000
Fixed rate mortgage notes payable \| 4.67% Construction To Permanent Loan Due 2035
Debt Instrument [Line Items]
Fixed rate mortgages								23,332,000				0
Fixed rate mortgage notes payable \| 4.88% due September 2032
Debt Instrument [Line Items]
Fixed rate mortgages								69,691,000				71,211,000
Fixed rate mortgage notes payable \| 3.75% due December 2032
Debt Instrument [Line Items]
Fixed rate mortgages								58,523,000				60,000,000
Fixed rate mortgage notes payable \| 3.69% Due March 2030
Debt Instrument [Line Items]
Fixed rate mortgages								27,060,000				27,884,000
Fixed rate mortgage notes payable \| 3.99% Due April 2030
Debt Instrument [Line Items]
Fixed rate mortgages								14,526,000				14,950,000
Fixed rate mortgage notes payable \| 4.41% mortgage maturing 2033
Debt Instrument [Line Items]
Fixed rate mortgages								31,941,000				0
Fixed rate mortgage notes payable \| 4.69% fixed rate loan maturing in 2034
Debt Instrument [Line Items]
Fixed rate mortgages								22,900,000				0
Variable rate loans payable
Debt Instrument [Line Items]
Variable rate debt								$ 122,000,000				75,135,000
Interest rate spread on LIBOR								3.84%
New Facility \| Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity								$ 400,000,000
Extension in line of credit facility period								1 year
Available borrowing capacity								$ 190,700,000
Outstanding line of credit								47,000,000
Letter of credit available borrowing capacity								184,600
New Facility \| Revolving Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Variable rate debt								47,000,000				61,000,000
Line of credit facility, maximum borrowing capacity								$ 325,000,000
Interest rate spread on LIBOR								1.35%
New Facility \| Revolving Credit Facility \| Line of Credit \| Minimum
Debt Instrument [Line Items]
Interest rate spread on LIBOR								1.35%
New Facility \| Revolving Credit Facility \| Line of Credit \| Maximum
Debt Instrument [Line Items]
Interest rate spread on LIBOR								1.95%
New Facility \| Revolving Credit Facility \| Term Facility
Debt Instrument [Line Items]
Interest rate spread on LIBOR								1.30%
New Facility \| Revolving Credit Facility \| Term Facility \| Minimum
Debt Instrument [Line Items]
Interest rate spread on LIBOR								1.30%
New Facility \| Revolving Credit Facility \| Term Facility \| Maximum
Debt Instrument [Line Items]
Interest rate spread on LIBOR								1.90%
New Facility \| Term Facility \| Line of Credit
Debt Instrument [Line Items]
Variable rate debt								$ 75,000,000				0
Line of credit facility, maximum borrowing capacity								$ 75,000,000
Interest rate spread on LIBOR								1.30%
Park Van Ness Loan
Debt Instrument [Line Items]
Fixed rate mortgages								$ 69,700,000
Guarantor obligations, maximum exposure								10,000,000
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 II Bank Loan \| Fixed rate mortgage notes payable
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure								8,800,000
Remaining loan balance								35,300,000
Burtonsville Fixed Rate Mortgage \| 3.39% due January 2032
Debt Instrument [Line Items]
Loan amount					$ 40,000,000
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
Glebe Road 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
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								4,000,000
Remaining loan balance								31,900,000
Broadlands Village \| 4.41% mortgage maturing 2033
Debt Instrument [Line Items]
Loan amount		$ 32,000,000
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
Debt Covenant
Debt Instrument [Line Items]
Carrying value of properties collateralized								$ 1,100,000,000				$ 1,000,000,000
Credit agreement leverage ratio								60.00%
Interest coverage ratio required minimum								2.0
Required fixed charge coverage ratio								1.4
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
Scenario, Forecast \| Park Van Ness Loan
Debt Instrument [Line Items]
Guarantor obligations, maximum exposure									$ 0	$ 3,300,000	$ 6,700,000
Scenario, Forecast \| Glebe Road 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													$ 71,580,000
Loan required periodic principal and interest payment								$ 413,500
Balloon payment to be paid								$ 39,600,000