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Real Estate Acquired - Additional Information (Detail) ft² in Thousands |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
May 09, 2018 shares | Dec. 31, 2018 USD ($) ft² apartment | Sep. 30, 2018 USD ($) | May 31, 2018 USD ($) a shares | Mar. 31, 2017 USD ($) | Jan. 31, 2017 USD ($) | Aug. 31, 2016 USD ($) a | Sep. 30, 2015 USD ($) | Dec. 31, 2014 USD ($) | Aug. 31, 2014 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2018 USD ($) ft² apartment | Dec. 31, 2017 USD ($) property | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire real estate |  |  |  |  |  |  |  |  |  |  |  | $ 12,883,000 | $ 17,653,000 | $ 15,564,000 |
Acquisition costs |  |  |  | $ 200,000 |  |  |  |  |  |  |  |  |  | 60,400 |
Rent expense |  |  |  |  |  |  |  |  |  |  |  | 779,800 | $ 774,700 | 843,300 |
Area of real estate property (in square feet) | a |  |  |  | 13,700 |  |  | 13,700 |  |  |  |  |  |  |  |
Number of properties acquired | property |  |  |  |  |  |  |  |  |  |  |  |  | 1 |  |
Purchase price allocated to land |  | $ 488,918,000 |  |  |  |  |  |  |  |  |  | 488,918,000 |  |  |
Purchase price allocated to buildings |  | 1,273,275,000 |  |  |  |  |  |  |  |  |  | 1,273,275,000 |  |  |
Below market lease, carrying amount |  | 24,800,000 |  |  |  |  |  |  |  |  |  | 24,800,000 | $ 25,100,000 |  |
Below market lease, accumulated amortization |  | 13,100,000 |  |  |  |  |  |  |  |  |  | 13,100,000 | 11,800,000 |  |
Accretion income, intangible liabilities |  |  |  |  |  |  |  |  |  |  |  | 1,700,000 | 1,700,000 | 1,800,000 |
Total Purchase Price |  | 1,948,165,000 |  |  |  |  |  |  |  |  | $ 1,700,813,000 | 1,948,165,000 | 1,803,200,000 | 1,700,813,000 | $ 1,622,710,000
Lease acquisition costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets gross carrying amount |  | 12,500,000 |  |  |  |  |  |  |  |  |  | 12,500,000 | 12,300,000 |  |
Intangible assets, accumulated amortization |  | 8,100,000 |  |  |  |  |  |  |  |  |  | 8,100,000 | 7,500,000 |  |
Amortization expense |  |  |  |  |  |  |  |  |  |  |  | $ 1,300,000 | 1,100,000 | 1,000,000 |
Weighted-average amortization period |  |  |  |  |  |  |  |  |  |  |  | 4 years 1 month 6 days |  |  |
Above Market Rent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets gross carrying amount |  | 800,000 |  |  |  |  |  |  |  |  |  | $ 800,000 | 600,000 |  |
Intangible assets, accumulated amortization |  | 100,000 |  |  |  |  |  |  |  |  |  | 100,000 | 39,500 |  |
Amortization expense |  |  |  |  |  |  |  |  |  |  |  | $ 110,500 | 31,600 | 1,500 |
Weighted-average amortization period |  |  |  |  |  |  |  |  |  |  |  | 7 years 2 months 12 days |  |  |
Below Market Rent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted-average amortization period |  |  |  |  |  |  |  |  |  |  |  | 5 years 6 months |  |  |
750 N. Glebe Road |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire real estate |  |  |  |  |  |  |  |  |  | $ 40,000,000 |  |  |  |  |
Acquisition costs |  |  |  |  |  |  |  |  |  | $ 400,000 |  |  |  |  |
730 N. Glebe Road |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire real estate |  |  |  |  |  |  |  |  | $ 2,800,000 |  |  |  |  |  |
Acquisition costs |  |  |  |  |  |  |  |  | $ 40,400 |  |  |  |  |  |
726 N Glebe Road |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire real estate |  |  |  |  |  |  |  | $ 4,000,000 |  |  |  |  |  |  |
Acquisition costs |  |  |  |  |  |  |  | $ 100,000 |  |  |  |  |  |  |
700 N. Glebe Road |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire real estate |  |  |  |  |  |  | $ 7,200,000 |  |  |  |  |  |  |  |
Purchase price allocated to land |  |  |  |  |  |  |  |  |  |  | 7,236,000 |  |  | 7,236,000 |
Purchase price allocated to buildings |  |  |  |  |  |  |  |  |  |  | 0 |  |  | 0 |
Total Purchase Price |  |  |  |  |  |  |  |  |  |  | 7,236,000 |  |  | 7,236,000 |
700 N. Glebe Road | In-place Leases |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets |  |  |  |  |  |  |  |  |  |  | 0 |  |  | 0 |
700 N. Glebe Road | Above Market Rent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets |  |  |  |  |  |  |  |  |  |  | 0 |  |  | 0 |
700 N. Glebe Road | Below Market Rent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets |  |  |  |  |  |  |  |  |  |  | 0 |  |  | 0 |
Thruway Pad |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire real estate |  |  |  |  |  |  | 3,100,000 |  |  |  |  |  |  |  |
Acquisition costs |  |  |  |  |  |  | $ 60,400 |  |  |  |  |  |  |  |
Purchase price allocated to land |  |  |  |  |  |  |  |  |  |  | 2,196,000 |  |  | 2,196,000 |
Purchase price allocated to buildings |  |  |  |  |  |  |  |  |  |  | 874,000 |  |  | 874,000 |
Total Purchase Price |  |  |  |  |  |  |  |  |  |  | 3,100,000 |  |  | 3,100,000 |
Thruway Pad | In-place Leases |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets |  |  |  |  |  |  |  |  |  |  | 93,000 |  |  | 93,000 |
Thruway Pad | Above Market Rent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets |  |  |  |  |  |  |  |  |  |  | 0 |  |  | 0 |
Thruway Pad | Below Market Rent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets |  |  |  |  |  |  |  |  |  |  | 63,000 |  |  | 63,000 |
Beacon Center |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire real estate |  |  |  |  |  |  |  |  |  |  | 22,700,000 |  |  |  |
Rent expense |  |  |  |  |  |  |  |  |  |  | 60,000 |  |  |  |
Mortgage loans on real estate, period increase (decrease) |  |  |  |  |  |  |  |  |  |  | 11,250,000 |  |  |  |
Southdale |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire real estate |  |  |  |  |  |  |  |  |  |  | 15,300,000 |  |  |  |
Rent expense |  |  |  |  |  |  |  |  |  |  | 60,000 |  |  |  |
Burtonsville Town Square |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire real estate |  |  |  |  |  | $ 76,400,000 |  |  |  |  |  |  | 76,400,000 |  |
Purchase price allocated to land |  |  |  |  |  |  |  |  |  |  |  |  | 28,400,000 |  |
Purchase price allocated to buildings |  |  |  |  |  |  |  |  |  |  |  |  | 45,800,000 |  |
Burtonsville Town Square | In-place Leases |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets |  |  |  |  |  |  |  |  |  |  |  |  | 2,200,000 |  |
Burtonsville Town Square | Above Market Rent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets |  |  |  |  |  |  |  |  |  |  |  |  | 600,000 |  |
Burtonsville Town Square | Below Market Rent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets |  |  |  |  |  |  |  |  |  |  |  |  | $ 600,000 |  |
2016 Acquisitions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire real estate |  |  |  |  |  |  |  |  |  |  |  |  |  | 10,300,000 |
Purchase price allocated to land |  |  |  |  |  |  |  |  |  |  | 9,432,000 |  |  | 9,432,000 |
Purchase price allocated to buildings |  |  |  |  |  |  |  |  |  |  | 874,000 |  |  | 874,000 |
Total Purchase Price |  |  |  |  |  |  |  |  |  |  | 10,336,000 |  |  | 10,336,000 |
2016 Acquisitions | In-place Leases |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets |  |  |  |  |  |  |  |  |  |  | 93,000 |  |  | 93,000 |
2016 Acquisitions | Above Market Rent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets |  |  |  |  |  |  |  |  |  |  | 0 |  |  | 0 |
2016 Acquisitions | Below Market Rent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible assets |  |  |  |  |  |  |  |  |  |  | $ 63,000 |  |  | $ 63,000 |
Olney |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire real estate |  |  |  |  | $ 3,100,000 |  |  |  |  |  |  |  |  |  |
Rent expense |  |  |  |  | $ 56,000 |  |  |  |  |  |  |  |  |  |
2018 Acquisitions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase price allocated to land |  | 47,438,000 |  |  |  |  |  |  |  |  |  | $ 47,438,000 |  |  |
Purchase price allocated to buildings |  | 979,000 |  |  |  |  |  |  |  |  |  | 979,000 |  |  |
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 |  |  |
Ashbrook Marketplace |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Limited partnership units issued |  |  |  | $ 8,776,000 |  |  |  |  |  |  |  |  |  |  |
7316 Wisconsin Avenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire real estate |  |  | $ 35,500,000 |  |  |  |  |  |  |  |  |  |  |  |
Acquisition costs |  |  | $ 700,000 |  |  |  |  |  |  |  |  |  |  |  |
Parcel Adjacent to 7316 Wisconsin Avenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to acquire real estate |  | $ 4,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |
7316 Wisconsin Avenue Retail Space |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Area of real estate property (in acres) | ft² |  | 10 |  |  |  |  |  |  |  |  |  | 10 |  |  |
BF Saul Real Estate Investment Trust | Ashbrook Marketplace |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Limited partnership units issued (in shares) | shares | 176,680 |  |  | 176,680 |  |  |  |  |  |  |  |  |  |  |
Maximum | 7316 Wisconsin Avenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of units in real estate property | apartment |  | 325 |  |  |  |  |  |  |  |  |  | 325 |  |  |
Maximum | 7316 Wisconsin Avenue Additional Apartment Units |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of units in real estate property | apartment |  | 40 |  |  |  |  |  |  |  |  |  | 40 |  |  |

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- Definition Amount before accumulated deprecation and depletion of real estate held for productive use and additions or improvements to real estate held for productive use, examples include, but are not limited to, walkways, driveways, fences, and parking lots. Excludes land held for sale. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6812-107765 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition The number of units in a real estate property owned as of the balance sheet date. + References No definition available. + Details
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- Definition Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41499-112717 + Details
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