Real Estate Activities Real Estate Activities - Acquisitions and Dispositions(Details) - USD ($)	1 Months Ended					3 Months Ended		9 Months Ended
	Sep. 30, 2017	Aug. 31, 2017	Jun. 30, 2017	Aug. 31, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Real Estate Property Sale Price	$ 61,500,000
Proceeds from Real Estate and Real Estate Joint Ventures	30,300,000
Repayments of Secured Debt								$ 150,000,000	$ 280,327,000
Deferred costs and other assets, net	107,304,000					$ 107,304,000		107,304,000		$ 93,800,000
Gains (Losses) on Sales of Investment Real Estate						(9,000)	$ 31,000	(374,000)	22,953,000
Gain (Loss) on Sale of Properties								112,000	22,962,000
Asset Impairment Charges						$ 1,825,000	$ 9,865,000	55,742,000	$ 24,589,000
Beaver Valley Mall [Member]
Property, Plant and Equipment [Line Items]
Fair Value Inputs, Cap Rate				15.60%
Proceeds from Sale of Real Estate				$ 24,200
Gain Loss On Sale Of Real Estate				$ 0
Crossroads Mall [Member]
Property, Plant and Equipment [Line Items]
Fair Value Inputs, Cap Rate					15.50%
Proceeds from Sale of Real Estate					$ 24,800
Gain Loss On Sale Of Real Estate					$ 0
Beaver Valley Mall [Member]
Property, Plant and Equipment [Line Items]
Proceeds from Sale of Real Estate								4,200,000
Gain Loss On Sale Of Real Estate								500,000
Vacant anchors purchased from Macys at Moorestown Valley View and Valley Malls [Member]
Property, Plant and Equipment [Line Items]
Payments to Acquire Real Estate								$ 13,900,000
Gainesville Land [Member]
Property, Plant and Equipment [Line Items]
Impairment Of Assets	$ 1,300,000
Logan Valley Mall [Member]
Property, Plant and Equipment [Line Items]
Fair Value Inputs, Cap Rate		16.50%
Proceeds from Sale of Real Estate		$ 33,200
Gain Loss On Sale Of Real Estate		$ 0
Impairment Of Assets			$ 38,700,000