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Notes Payable, Revolving Credit Facility, Interest and Amortization of Deferred Debt Costs - Narrative (Details) - USD ($) |  |  |  |  | 3 Months Ended | 9 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Nov. 20, 2017 | Sep. 01, 2017 | Aug. 14, 2017 | Jan. 18, 2017 | Dec. 31, 2021 | Sep. 30, 2018 | Oct. 01, 2021 | Oct. 01, 2020 | Oct. 01, 2019 | Jan. 26, 2018 | Dec. 31, 2017 | Jan. 31, 2017
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt outstanding with fixed-rate |  |  |  |  |  | $ 870,901,000 |  |  |  |  | $ 890,400,000 |
Principal amount |  |  |  |  |  | 1,022,901,000 |  |  |  |  | 965,500,000 |
Debt outstanding with variable-rate |  |  |  |  |  | 152,000,000 |  |  |  |  | 75,100,000 |
Outstanding line of credit |  |  |  |  |  | 75,200,000 |  |  |  |  | 60,734,000 |
Carrying value of properties collateralizing mortgage notes |  |  |  |  |  | 965,600,000 |  |  |  |  | 1,000,000,000 |
Final payment amount |  |  |  |  |  | 760,955,000 |  |  |  |  |  |
Principal amount |  |  |  |  |  | 1,012,912,000 |  |  |  |  |  |
Construction loan |  |  |  |  |  | 861,897,000 |  |  |  |  | 897,888,000 |
Unsecured Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Outstanding line of credit |  |  |  |  |  | 77,000,000 |  |  |  |  | $ 61,000,000 |
Line of credit facility, remaining borrowing capacity |  |  |  |  |  | 161,100,000 |  |  |  |  |  |
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 |  |
New Facility | Term Loan | Line of Credit |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Outstanding line of credit |  |  |  |  |  | $ 75,000,000 |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  |  |  |  |  |  |  |  | 75,000,000 |  |
New Facility | Term Loan | LIBOR |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest rate spread on LIBOR (percentage) |  |  |  |  |  | 1.30% |  |  |  |  |  |
New Facility | Term Loan | LIBOR | Minimum |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest rate spread on LIBOR (percentage) |  |  |  |  |  | 1.30% |  |  |  |  |  |
New Facility | Term Loan | LIBOR | Maximum |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest rate spread on LIBOR (percentage) |  |  |  |  |  | 1.90% |  |  |  |  |  |
New Facility | Revolving Line | Line of Credit |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  |  |  |  |  |  |  |  | $ 325,000,000 |  |
New Facility | Revolving Line | LIBOR |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest rate spread on LIBOR (percentage) |  |  |  |  |  | 1.35% |  |  |  |  |  |
New Facility | Revolving Line | LIBOR | Minimum |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest rate spread on LIBOR (percentage) |  |  |  |  |  | 1.35% |  |  |  |  |  |
New Facility | Revolving Line | LIBOR | Maximum |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest rate spread on LIBOR (percentage) |  |  |  |  |  | 1.95% |  |  |  |  |  |
Burtonsville Fixed Rate Mortgage |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, term |  |  |  | 15 years |  |  |  |  |  |  |  |
Construction-to-permanent loan |  |  |  | $ 40,000,000 |  |  |  |  |  |  |  | $ 40,000,000.0
Interest rate, stated percentage |  |  |  | 3.39% |  |  |  |  |  |  |  |
Periodic payment |  |  |  | $ 197,900 |  |  |  |  |  |  |  |
Amortization period |  |  |  | 25 years |  |  |  |  |  |  |  |
Final payment amount |  |  |  | $ 20,300,000 |  |  |  |  |  |  |  |
Glebe Road Construction Loan |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Construction-to-permanent loan |  |  | $ 157,000,000.0 |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  | 4.67% |  |  |  |  |  |  |  |  |
Amortization period |  |  | 25 years |  |  |  |  |  |  |  |  |
Principal amount |  |  |  |  |  | $ 2,900,000 |  |  |  |  |  |
Park Van Ness Loan |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  | 4.88% |  |  |  |  |  |  |  |  |  |
Periodic payment |  | $ 413,460 |  |  |  |  |  |  |  |  |  |
Amortization period |  | 25 years |  |  |  |  |  |  |  |  |  |
Final payment amount |  | $ 39,600,000 |  |  |  |  |  |  |  |  |  |
Guarantor obligations, maximum exposure |  |  |  |  |  | 10,000,000 |  |  |  |  |  |
Construction loan |  |  |  |  |  | 70,100,000 |  |  |  |  |  |
Washington Square 2017 Mortgage | 3.75% due December 2032 |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, term | 15 years |  |  |  |  |  |  |  |  |  |  |
Construction-to-permanent loan | $ 60,000,000 |  |  |  |  |  |  |  |  |  |  |
Interest rate, stated percentage | 3.75% |  |  |  |  |  |  |  |  |  |  |
Periodic payment | $ 308,500 |  |  |  |  |  |  |  |  |  |  |
Amortization period | 25 years |  |  |  |  |  |  |  |  |  |  |
Final payment amount | $ 31,100,000 |  |  |  |  |  |  |  |  |  |  |
Kentlands Square II Bank Loan | Fixed Rate Mortgage Notes Payable |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Principal amount |  |  |  |  |  | 35,600,000 |  |  |  |  |  |
Guarantor obligations, maximum exposure |  |  |  |  |  | $ 9,200,000 |  |  |  |  |  |
Forecast | Glebe Road Construction Loan |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Periodic payment |  |  |  |  | $ 887,900 |  |  |  |  |  |  |
Forecast | Park Van Ness Loan |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Guarantor obligations, maximum exposure |  |  |  |  |  |  | $ 0 | $ 3,300,000 | $ 6,700,000 |  |  |
Washington Square |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Principal amount | $ 28,100,000 |  |  |  |  |  |  |  |  |  |  |

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