Real Estate Activities (Dispositions of Assets) (Detail) - USD ($)	1 Months Ended							12 Months Ended
	Sep. 30, 2017	Jan. 31, 2017	Aug. 31, 2016	Mar. 31, 2016	Feb. 29, 2016	Oct. 31, 2015	Aug. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Disposals [Line Items]
Proceeds from Sale of Land Held-for-use								$ 6,400,000
Gain (Loss) on Sale of Properties								1,300,000
Sale Price	$ 61,500,000
Voorhees office building [Member]
Disposals [Line Items]
Sale Price								9,300,000
Gain/ (Loss)								900,000
Parcel And Land Improvements [Member]
Disposals [Line Items]
Sale Price									$ 5,100,000
Wiregrass Commons [Member]
Disposals [Line Items]
Loans Receivable with Fixed Rates of Interest				$ 17,000,000				$ 16,500,000
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]
Capitalization Rate							17.50%
Sale Price			$ 20,000,000				$ 23,000,000
Gain/ (Loss)			$ (100,000)				$ 0
Voorhees Town Center [Member] | Mall [Member]
Disposals [Line Items]
Capitalization Rate						10.30%
Sale Price						$ 13,400,000
Gain/ (Loss)						$ 0