Discontinued Operations and Assets Held for Sale (Aggregate Balance Sheet of Properties Held for Sale Included in Discontinued Operations) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) buildings	Dec. 31, 2016 USD ($)
Discontinued Operations
Discontinued Operation, Number of Real Estate Properties Held for Sale | buildings	0
Continuing Operations, Number of In-service Properties Held for Sale | buildings	1
Land and Land Improvements Held-for-Sale	$ 8,157	$ 122,513
Buildings and Tenant Improvements Held-for-Sale	10,505	1,255,963
Inventory, Land Held-for-sale	0	22,657
Accumulated Depreciation of Assets Held-for-Sale	(2,553)	(259,266)
Lease Related Assets, Held for Sale	862	86,613
Other assets	579	95,778
Total assets held-for-sale	17,550	1,324,258
Total liabilities held-for-sale	$ 1,163	56,291
Discontinued Operations [Member]
Discontinued Operations
Land and Land Improvements Held-for-Sale		118,882
Buildings and Tenant Improvements Held-for-Sale		1,218,468
Inventory, Land Held-for-sale		0
Accumulated Depreciation of Assets Held-for-Sale		(240,685)
Lease Related Assets, Held for Sale		83,522
Other assets		92,444
Total assets held-for-sale		1,272,631
Total liabilities held-for-sale		54,630
Continuing Operations [Member]
Discontinued Operations
Land and Land Improvements Held-for-Sale		3,631
Buildings and Tenant Improvements Held-for-Sale		37,495
Inventory, Land Held-for-sale		22,657
Accumulated Depreciation of Assets Held-for-Sale		(18,581)
Lease Related Assets, Held for Sale		3,091
Other assets		3,334
Total assets held-for-sale		51,627
Total liabilities held-for-sale		$ 1,661