Real Estate Assets, 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, 2017 USD ($) a buildings	Dec. 31, 2016 USD ($)
Held for Sale Assets in Continuing and Discontinued Operations
Number of Real Estate Properties Held for Sale | buildings	2
Discontinued Operation, Number of Real Estate Properties Held for Sale | buildings	1
Area of Land | a	12
Land and Land Improvements Held-for-Sale	$ 21,719	$ 122,513
Buildings and Tenant Improvements Held-for-Sale	51,356	1,255,963
Inventory, Land Held-for-sale	4,909	22,657
Accumulated Depreciation of Assets Held-for-Sale	(21,736)	(259,266)
Lease Related Assets, Held for Sale	2,983	86,613
Other assets	4,373	95,778
Total assets held-for-sale	63,604	1,324,258
Total liabilities held-for-sale	2,653	56,291
Continuing Operations [Member]
Held for Sale Assets in Continuing and Discontinued Operations
Land and Land Improvements Held-for-Sale	8,157	3,631
Buildings and Tenant Improvements Held-for-Sale	10,505	37,495
Inventory, Land Held-for-sale	4,909	22,657
Accumulated Depreciation of Assets Held-for-Sale	(2,553)	(18,581)
Lease Related Assets, Held for Sale	862	3,091
Other assets	591	3,334
Total assets held-for-sale	22,471	51,627
Total liabilities held-for-sale	1,169	1,661
Discontinued Operations [Member]
Held for Sale Assets in Continuing and Discontinued Operations
Land and Land Improvements Held-for-Sale	13,562	118,882
Buildings and Tenant Improvements Held-for-Sale	40,851	1,218,468
Inventory, Land Held-for-sale	0	0
Accumulated Depreciation of Assets Held-for-Sale	(19,183)	(240,685)
Lease Related Assets, Held for Sale	2,121	83,522
Other assets	3,782	92,444
Total assets held-for-sale	41,133	1,272,631
Total liabilities held-for-sale	$ 1,484	$ 54,630