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Real Estate - Additional Information (Detail) | 1 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 31, 2018 USD ($) | Sep. 30, 2018 USD ($) | May 31, 2018 USD ($) a shares | Aug. 31, 2016 shares | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Sep. 01, 2019 USD ($) | Dec. 31, 2017 USD ($)
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Acquisition costs |  |  | $ 200,000 |  |  |  |  |  |
Development in process |  |  |  |  | $ 69,477,000 | $ 335,644,000 |  |  |
Total purchase price | $ 1,948,165,000 |  |  |  | 2,124,796,000 | 2,081,597,000 | $ 1,948,165,000 |  | $ 1,803,200,000
Below market lease, carrying amount |  |  |  |  | 23,700,000 | 24,100,000 |  |  |
Below market lease, accumulated amortization |  |  |  |  | 15,000,000.0 | 13,900,000 |  |  |
Accretion income, intangible liabilities |  |  |  |  | 1,400,000 | 1,500,000 | 1,700,000 |  |
Above Market Rent |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Intangible assets gross carrying amount |  |  |  |  | 600,000 | 600,000 |  |  |
Intangible assets, accumulated amortization |  |  |  |  | 128,900 | 108,300 |  |  |
Amortization expense |  |  |  |  | $ 43,600 | 109,600 | 110,500 |  |
Weighted-average amortization period |  |  |  |  | 7 years |  |  |  |
Below Market Rent |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Weighted-average amortization period |  |  |  |  | 5 years |  |  |  |
Lease acquisition costs |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Intangible assets gross carrying amount |  |  |  |  | $ 11,000,000.0 | 11,700,000 |  |  |
Intangible assets, accumulated amortization |  |  |  |  | 8,600,000 | 8,500,000 |  |  |
Amortization expense |  |  |  |  | $ 600,000 | 900,000 | 1,300,000 |  |
Weighted-average amortization period |  |  |  |  | 4 years 3 months 18 days |  |  |  |
Ashbrook Marketplace |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Limited partnership units issued |  |  | $ 8,800,000 |  |  |  | 8,776,000 |  |
Development in process |  |  |  |  | $ 153,000 | 19,128,000 |  |  |
Hampden House (formerly 7316 Wisconsin Avenue) |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Acquisition costs |  | $ 700,000 |  |  |  |  |  |  |
Payments to acquire real estate |  | $ 35,500,000 |  |  |  |  |  |  |
Development in process |  |  |  |  | $ 50,723,000 | $ 44,638,000 |  | $ 42,600,000 |
Parcel Adjacent to 7316 Wisconsin Avenue |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Payments to acquire real estate | 4,500,000 |  |  |  |  |  |  |  |
BF Saul Real Estate Investment Trust | Ashbrook Marketplace |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Limited partnership units issued (in shares) | shares |  |  | 176,680 | 176,680 |  |  |  |  |
Area of land | a |  |  | 13.7 |  |  |  |  |  |
2018 Acquisitions |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Total purchase price | 49,450,000 |  |  |  |  |  | 49,450,000 |  |
2018 Acquisitions | In-place Leases |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Intangible assets | 886,000 |  |  |  |  |  | 886,000 |  |
2018 Acquisitions | Above Market Rent |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Intangible assets | 168,000 |  |  |  |  |  | 168,000 |  |
2018 Acquisitions | Below Market Rent |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |
Intangible assets | $ 21,000 |  |  |  |  |  | $ 21,000 |  |

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