Real Estate Activities (Dispositions of Assets) (Detail) - USD ($)	1 Months Ended				12 Months Ended
	Jan. 31, 2017	Aug. 31, 2016	Mar. 31, 2016	Feb. 29, 2016	Dec. 31, 2018
Disposals [Line Items]
Proceeds from Sale of Land Held-for-use					$ 6,400,000
Gain (Loss) on Sale of Properties					1,300,000
Voorhees office building [Member]
Disposals [Line Items]
Sale Price					9,300,000
Gain/ (Loss)					900,000
Wiregrass Commons [Member]
Disposals [Line Items]
Loans Receivable with Fixed Rates of Interest			$ 17,000,000		$ 16,100,000
Beaver Valley Mall [Member] | Mall [Member]
Disposals [Line Items]
Capitalization Rate	15.60%
Sale Price	$ 24,200,000
Gain/ (Loss)	$ 0
Crossroads Mall [Member] | Mall [Member]
Disposals [Line Items]
Capitalization Rate	15.50%
Sale Price	$ 24,800,000
Gain/ (Loss)	$ 0
Logan Valley Mall [Member] | Mall [Member]
Disposals [Line Items]
Capitalization Rate	16.50%
Sale Price	$ 33,200,000
Gain/ (Loss)	$ 0
Palmer Park Mall [Member] | Mall [Member]
Disposals [Line Items]
Capitalization Rate				13.60%
Sale Price				$ 18,000,000
Gain/ (Loss)				$ 100,000
Gadsden Mall, New River Valley Mall and Wiregrass Commons Mall [Member] | Mall [Member]
Disposals [Line Items]
Capitalization Rate			17.40%
Sale Price			$ 66,000,000
Gain/ (Loss)			$ 1,600,000
Lycoming Mall [Member] | Mall [Member]
Disposals [Line Items]
Capitalization Rate			18.00%
Sale Price			$ 26,400,000
Gain/ (Loss)			$ 300,000
Walnut and Chestnut Street Retail [Member] | Walnut and Chestnut Street Retail [Member]
Disposals [Line Items]
Capitalization Rate		3.20%
Sale Price		$ 45,000,000
Gain/ (Loss)		$ 20,300,000
Washington Crown Center [Member] | Mall [Member]
Disposals [Line Items]
Capitalization Rate		14.50%
Uniontown Mall [Member] | Mall [Member]
Disposals [Line Items]
Sale Price		$ 20,000,000
Gain/ (Loss)		$ (100,000)