Real Estate (Schedule of Transactions Related to Properties Held for Sale/Conveyance) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Real Estate Properties [Line Items]
Gain on Sale	$ 4,396,000	$ 2,942,000	$ 4,864,000
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Real Estate Properties [Line Items]
Sales Price	21,928,000	19,378,000	19,600,000
Gain on Sale	$ 4,396,000	$ 2,942,000	$ 4,864,000
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Metro Square [Member]
Real Estate Properties [Line Items]
Location	Owings Mills, MD
Date Sold	Jul. 09, 2020
Sales Price	$ 4,288,000
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Maxatawny Marketplace [Member]
Real Estate Properties [Line Items]
Location		Maxatawny, PA
Date Sold		Feb. 15, 2019
Sales Price		$ 10,330,000
Gain on Sale		$ 101,000
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Mechanicsburg Center [Member]
Real Estate Properties [Line Items]
Location			Mechanicsburg, PA
Date Sold			Aug. 28, 2018
Sales Price			$ 16,100,000
Gain on Sale			$ 4,864,000
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | West Bridgewater Plaza [Member]
Real Estate Properties [Line Items]
Location			West Bridgewater, MA
Date Sold			Sep. 28, 2018
Sales Price			$ 3,500,000
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Oakland Mills Outparcel Building [Member]
Real Estate Properties [Line Items]
Location	Columbia, MD
Date Sold	Sep. 17, 2020
Sales Price	$ 1,050,000
Gain on Sale	$ 643,000
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Fort Washington Center [Member]
Real Estate Properties [Line Items]
Location		Fort Washington, PA
Date Sold		Jun. 26, 2019
Sales Price		$ 9,048,000
Gain on Sale		$ 2,841,000
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Glen Allen Shopping Center [Member]
Real Estate Properties [Line Items]
Location	Glen Allen, VA
Date Sold	Oct. 08, 2020
Sales Price	$ 8,540,000
Gain on Sale	$ 1,780,000
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Pine Grove Plaza Outparcel Building [Member]
Real Estate Properties [Line Items]
Location	Brown Mills, NJ
Date Sold	Nov. 02, 2020
Sales Price	$ 1,100,000
Gain on Sale	$ 565,000
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Suffolk Plaza [Member]
Real Estate Properties [Line Items]
Location	Suffolk, VA
Date Sold	Dec. 10, 2020
Sales Price	$ 6,950,000
Gain on Sale	$ 1,408,000