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Real Estate Transactions - Narrative (Details)		1 Months Ended			3 Months Ended		12 Months Ended
	Jan. 12, 2018 USD ($) ft² apartment	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Jan. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) property
Business Acquisition [Line Items]
Payments to acquire real estate					$ 3,483,000	$ 4,557,000
Rent expense					$ 195,100	$ 188,100
Number of properties purchased | property							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
Portion of purchase price allocated to land							$ 28,400,000
Portion of purchase price allocated to buildings							45,800,000
Olney
Business Acquisition [Line Items]
Payments to acquire real estate			$ 3,100,000
Rent expense			$ 56,000
Great Eastern Shopping Center
Business Acquisition [Line Items]
Second trust financing		$ 1,280,000
Second trust financing, stated interest rate		6.00%
Optional extension period		6 months
7316 Wisconsin Avenue
Business Acquisition [Line Items]
Payments to acquire real estate	$ 35,500,000
Property acquisition costs	700,000
Earnest money deposits	$ 3,500,000
Number of units in real estate property | apartment	325
7316 Wisconsin Avenue Retail Space
Business Acquisition [Line Items]
Area of real estate property (in square feet) | ft²	10,000
Leases, Acquired-in-Place | Burtonsville Town Square
Business Acquisition [Line Items]
Portion of purchase price allocated to leases acquired in place							(2,200,000)
Below Market Leases | Burtonsville Town Square
Business Acquisition [Line Items]
Portion of purchase price allocated to leases acquired in place							(600,000)
Below Market Leases | Burtonsville Town Square
Business Acquisition [Line Items]
Portion of purchase price allocated to leases acquired in place							$ (600,000)

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