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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 |  |  |  |  |  |  |

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