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Real Estate Transactions - Narrative (Details) ft² in Thousands |  | 1 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
May 09, 2018 shares | Sep. 30, 2018 USD ($) ft² apartment | May 31, 2018 USD ($) a shares | Sep. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Jan. 31, 2017 USD ($) | Aug. 31, 2016 a | Sep. 30, 2018 USD ($) ft² property apartment | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) property | Jan. 18, 2017 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Payments to acquire real estate |  |  |  |  |  |  |  | $ 6,586,000 | $ 12,389,000 |  |
Rent expense |  |  |  |  |  |  |  | $ 590,000 | 578,200 |  |
Number of properties purchased | property |  |  |  |  |  |  |  | 1 |  | 1 |
Great Eastern Shopping Center |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale of real estate |  |  |  | $ 8,500,000 |  |  |  |  |  |  |
Gains (losses) on sales of investment real estate |  |  | $ 500,000 |  |  |  |  |  |  |  |
Burtonsville Town Square |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Payments to acquire real estate |  |  |  |  |  | $ 76,400,000 |  |  |  | $ 76,400,000 |
Portion of purchase price allocated to land |  |  |  |  |  |  |  |  |  | 28,400,000 |
Portion of purchase price allocated to buildings |  |  |  |  |  |  |  |  |  | 45,800,000 |
Portion of purchase price allocated to below market leases |  |  |  |  |  |  |  |  |  | (600,000) |
Olney |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Payments to acquire real estate |  |  |  |  | $ 3,100,000 |  |  |  |  |  |
Rent expense |  |  |  |  | $ 56,000 |  |  |  |  |  |
Ashbrook Marketplace |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Area of real estate acquired (in acres) | a |  |  | 13.7 |  |  |  |  |  |  |  |
Additions to real estate investments, noncash |  |  | $ 8,800,000 |  |  |  |  | $ 8,776,000 |  |  |
Acquisition costs |  |  | $ 200,000 |  |  |  |  |  |  |  |
7316 Wisconsin Avenue |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Payments to acquire real estate |  | $ 35,500,000 |  |  |  |  |  | 36,200,000 |  |  |
Portion of purchase price allocated to land |  | 34,200,000 |  |  |  |  |  | 34,200,000 |  |  |
Portion of purchase price allocated to buildings |  | 1,000,000 |  |  |  |  |  | 1,000,000 |  |  |
Property acquisition costs |  | $ 700,000 |  |  |  |  |  | $ 700,000 |  |  |
7316 Wisconsin Avenue Retail Space |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Area of real estate property (in square feet) | ft² |  | 10 |  |  |  |  |  | 10 |  |  |
Great Eastern Shopping Center |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Second trust financing |  | $ 1,275,000 |  | $ 1,280,000 |  |  |  | $ 1,275,000 | $ 1,280,000 |  |
Second trust financing, stated interest rate |  |  |  | 6.00% |  |  |  |  |  |  |
Leases, Acquired-in-Place | Burtonsville Town Square |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Portion of purchase price allocated to in-place leases and above market leases |  |  |  |  |  |  |  |  |  | 2,200,000 |
Leases, Acquired-in-Place | 7316 Wisconsin Avenue |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Portion of purchase price allocated to in-place leases and above market leases |  | 900,000 |  |  |  |  |  | 900,000 |  |  |
Above Market Leases | Burtonsville Town Square |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Portion of purchase price allocated to in-place leases and above market leases |  |  |  |  |  |  |  |  |  | $ 600,000 |
Above Market Leases | 7316 Wisconsin Avenue |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Portion of purchase price allocated to in-place leases and above market leases |  | $ 100,000 |  |  |  |  |  | $ 100,000 |  |  |
BF Saul Real Estate Investment Trust | Ashbrook Marketplace |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Limited partnership units issued for purchase (in shares) | shares | 176,680 |  | 176,680 |  |  |  |  |  |  |  |
Area of real estate acquired (in acres) | a |  |  |  |  |  |  | 13.7 |  |  |  |
Burtonsville Fixed Rate Mortgage |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Construction-to-permanent loan |  |  |  |  |  | $ 40,000,000.0 |  |  |  |  | $ 40,000,000
Maximum | 7316 Wisconsin Avenue |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of units in real estate property | apartment |  | 325 |  |  |  |  |  | 325 |  |  |

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