Discontinued Operations and Assets Held for Sale (Aggregate Balance Sheet of Properties Held for Sale Included in Discontinued Operations) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2015 USD ($) a buildings	Dec. 31, 2014 USD ($)
Held for Sale Assets in Continuing and Discontinued Operations
Land and Land Improvements Held-for-Sale		$ 148,268
Buildings and Tenant Improvements Held-for-Sale		758,323
Inventory, Land Held-for-sale		12,443
Accumulated Depreciation of Assets Held-for-Sale		(270,340)
Lease Related Assets, Held for Sale		48,320
Other assets		28,037
Total assets held-for-sale	$ 8,920	725,051
Disposal Groups, Secured Debt Related to Assets Held for Sale		40,764
Accrued expenses		5,413
Other liabilities		12,915
Total liabilities held-for-sale	$ 225	59,092
Discontinued Operation, Number of Real Estate Properties Held for Sale | buildings	0
Continuing Operations, Number of In-service Properties Held for Sale | buildings	1
Continuing Operations [Member]
Held for Sale Assets in Continuing and Discontinued Operations
Land and Land Improvements Held-for-Sale	$ 596	21,347
Buildings and Tenant Improvements Held-for-Sale	8,039	36,925
Inventory, Land Held-for-sale	6,349	12,443
Accumulated Depreciation of Assets Held-for-Sale	(6,283)	(23,071)
Lease Related Assets, Held for Sale	67	3,480
Other assets	152	562
Total assets held-for-sale	8,920	51,686
Disposal Groups, Secured Debt Related to Assets Held for Sale	0	0
Accrued expenses	183	233
Other liabilities	42	434
Total liabilities held-for-sale	$ 225	667
Discontinued Operations [Member]
Held for Sale Assets in Continuing and Discontinued Operations
Land and Land Improvements Held-for-Sale		126,921
Buildings and Tenant Improvements Held-for-Sale		721,398
Inventory, Land Held-for-sale		0
Accumulated Depreciation of Assets Held-for-Sale		(247,269)
Lease Related Assets, Held for Sale		44,840
Other assets		27,475
Total assets held-for-sale		673,365
Disposal Groups, Secured Debt Related to Assets Held for Sale		40,764
Accrued expenses		5,180
Other liabilities		12,481
Total liabilities held-for-sale		$ 58,425
Land [Member]
Held for Sale Assets in Continuing and Discontinued Operations
Area of Land | a	39