Real Estate Acquired and Sold - Narrative (Details)	1 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) ft²	Mar. 31, 2017 USD ($)	Jan. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) ft²	Nov. 30, 2016 USD ($)	Aug. 31, 2016 USD ($) property	Sep. 30, 2017 USD ($) ft² property	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) ft² property
Business Acquisition [Line Items]
Aggregate cost							$ 76,400,000		$ 10,300,000
Property acquisition costs				$ 60,400					$ 60,400
Rent expense							$ 578,200	$ 617,100
Number of properties purchased | property							1		2
Portion of purchase price allocated to land	$ 28,400,000			9,400,000			$ 28,400,000		$ 9,400,000
Portion of purchase price allocated to buildings	45,800,000			900,000			45,800,000		900,000
Portion of purchase price allocated to leases acquired in place	2,200,000			100,000			2,200,000		100,000
Portion of purchase price allocated to below market lease	(600,000)			(100,000)			(600,000)		$ (100,000)
Portion of purchase price allocated to above market lease	600,000						$ 600,000
Crosstown Business Center
Business Acquisition [Line Items]
Proceeds from sale of real estate				$ 5,400,000
Area of real estate property (in square feet) | ft²				197,100					197,100
Gains (losses) on sales of investment real estate				$ 1,000,000
Great Eastern Shopping Center
Business Acquisition [Line Items]
Proceeds from sale of real estate	$ 8,500,000
Area of real estate property (in square feet) | ft²	255,400						255,400
Gains (losses) on sales of investment real estate	$ 500,000
700 N. Glebe Road
Business Acquisition [Line Items]
Aggregate cost						$ 7,200,000
Number of contiguous properties | property						3
Thruway Pad
Business Acquisition [Line Items]
Aggregate cost						$ 3,100,000
Property acquisition costs						$ 60,400
Beacon Center
Business Acquisition [Line Items]
Aggregate cost					$ 22,500,000
Rent expense					60,000
Mortgage loans on real estate, period increase (decrease)					11,250,000
Southdale
Business Acquisition [Line Items]
Aggregate cost					15,000,000
Rent expense					$ 60,000
Burtonsville Town Square
Business Acquisition [Line Items]
Aggregate cost			$ 76,400,000
Olney
Business Acquisition [Line Items]
Aggregate cost		$ 3,000,000
Rent expense		$ 56,000
Great Eastern Shopping Center
Business Acquisition [Line Items]
Second trust financing	$ 1,275,000						$ 1,275,000
Second trust financing, stated interest rate	6.00%
Optional extension period	6 months