Real Estate Activities Real Estate Activities - Acquisitions and Dispositions(Details)	1 Months Ended	3 Months Ended			9 Months Ended
	Oct. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Property, Plant and Equipment [Line Items]
Repayments of Secured Debt					$ 272,776,000	$ 76,784,000
Liabilities related to future tenant improvements			$ 14,800,000
Deferred costs and other assets		$ 86,717,000			86,717,000		$ 87,017,000
Gains (Losses) on Sales of Investment Real Estate		12,386,000		$ (513,000)	12,386,000	(414,000)
Gain (Loss) on Sale of Properties					12,429,000	(414,000)
Asset Impairment Charges		51,412,000		$ 2,297,000	$ 86,319,000	$ 19,695,000
Voorhees Town Center [Member]
Property, Plant and Equipment [Line Items]
Asset Impairment Charges		$ 39,100,000
Springfield Town Center [Member]
Property, Plant and Equipment [Line Items]
Business Combination, Consideration			486,604,000
Assumption and payoff of long term debt			263,800,000
Land			119,912,000
Other significant noncash transaction, consideration given, OP Units		6,250,000
Buildings and Improvements, Gross			299,012,000
Improvements, Common Area			16,776,000
Land Improvements			35,565,000
Finite-Lived Intangible Asset, Acquired-in-Place Leases			18,123,000
Above Market Lease Gross			260,000
Below Market Lease, Acquired			(393,000)
Ground Leases, Gross			(5,882,000)
Deferred costs and other assets			3,231,000
Accrued Liabilities and Other Liabilities			$ 7,700,000
Uniontown Mall [Member]
Property, Plant and Equipment [Line Items]
Proceeds from Divestiture of Interest in Consolidated Subsidiaries		$ 23,000,000
Capitalization Rates		17.50%
Gain (Loss) on Sale of Properties		$ 0
Asset Impairment Charges		$ 7,500,000
Subsequent Event [Member] | Voorhees Town Center [Member]
Property, Plant and Equipment [Line Items]
Proceeds from Divestiture of Interest in Consolidated Subsidiaries	$ 13,400,000
Capitalization Rates	10.30%