Real Estate (Narrative) (Details)					3 Months Ended		9 Months Ended
	Oct. 08, 2020 USD ($)	Sep. 17, 2020 USD ($)	Jul. 23, 2020 USD ($) Memberworkforce $ / ft²	Jul. 09, 2020 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)
Real Estate Properties [Line Items]
Gain on sale of property					$ 679,000		$ 679,000	$ 2,942,000
Proceeds from sale of property held-for-sale				$ 4,300,000
Total revenues					$ 31,175,000	$ 35,912,000	102,280,000	$ 108,455,000
Impairment charges							7,607,000
Contract Termination [Member]
Real Estate Properties [Line Items]
Impairment charges							7,600,000
Other Income [Member] | Contract Termination [Member]
Real Estate Properties [Line Items]
Total revenues							$ 7,100,000
Outparcel Building Adjacent to Oakland Mills [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Real Estate Properties [Line Items]
Proceeds from sale of outparcel building		$ 1,100,000
Outparcel Building Adjacent to Oakland Mills [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Operating Income [Member]
Real Estate Properties [Line Items]
Gain on sale of property		$ 700,000
Glen Allen Shopping Center [Member] | Subsequent Event [Member]
Real Estate Properties [Line Items]
Proceeds from sale of property held-for-sale	$ 8,500,000
Commercial Lease Agreement [Member] | Development Asset [Member] | Government of the District of Columbia (“District”) [Member]
Real Estate Properties [Line Items]
Lease agreement entered date			Jul. 23, 2020
Number of member workforce planned to house new office headquarters | Memberworkforce			700
Term of lease			20 years 10 months
Lessor, operating lease not yet commenced, Description							The Company anticipates commencement of construction to occur in the first quarter of 2021 and currently estimates that the space will be delivered during the end of the fourth quarter 2022
Initial annual net rent amount to be paid upon completion of building			$ 5,400,000
Annual escalator percentage to be paid on each anniversary of rent commencement			2.50%
Operating costs, property taxes and amortization of tenant improvements			$ 8,100,000
Annual net rent amount to be paid upon completion of building			$ 13,500,000
Free rent period			10 months
Tenant credit against either annual rent or any other tenant payment obligations including tenant improvement costs, in excess of tenant improvement allowance			$ 6,800,000
Amortized over period			240 months
Commercial Lease Agreement [Member] | Development Asset [Member] | Government of the District of Columbia (“District”) [Member] | Between the 10th and 12th Lease Years [Member]
Real Estate Properties [Line Items]
Additional per rentable square foot tenant improvement allowance to be contributed | $ / ft²			9.38
Commercial Lease Agreement [Member] | Development Asset [Member] | Government of the District of Columbia (“District”) [Member] | Maximum [Member]
Real Estate Properties [Line Items]
Landlord contribution to be made per rentable square foot toward cost of tenant improvements | $ / ft²			155