Real Estate Activities Real Estate Activities - Acquisitions and Dispositions(Details) - Subsequent Event Type [Domain] $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Property, Plant and Equipment [Line Items]
Repayments of Secured Debt				$ 139,137
Liabilities related to future tenant improvements		$ 14,800
Deferred costs and other assets	$ 88,196			88,196		$ 87,017
Gains (Losses) on Sales of Investment Real Estate	$ 0		$ 99	$ 0	$ 99
Springfield Town Center [Member]
Property, Plant and Equipment [Line Items]
Business Combination, Consideration		486,604
Assumption and payoff of long term debt		263,800
Land		$ 119,912
Other significant noncash transaction, consideration given, OP Units		6,250,000
Buildings and Improvements, Gross		$ 299,012
Land Improvements		35,565
Finite-Lived Intangible Asset, Acquired-in-Place Leases		18,123
Above Market Lease Gross		260
Below Market Lease, Acquired		(393)
Ground Leases, Gross		(5,882)
Deferred costs and other assets		3,231
Accrued Liabilities and Other Liabilities		$ 7,700