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	6 Months Ended
	Jun. 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	10
Discontinued Operation, Number of Real Estate Properties Held for Sale | buildings	8
Area of Land | a	52
Land and Land Improvements Held-for-Sale	$ 37,082	$ 122,513
Buildings and Tenant Improvements Held-for-Sale	183,924	1,255,963
Inventory, Land Held-for-sale	17,082	22,657
Accumulated Depreciation of Assets Held-for-Sale	(43,775)	(259,266)
Lease Related Assets, Held for Sale	7,744	86,613
Other assets	11,597	95,778
Total assets held-for-sale	213,654	1,324,258
Accrued expenses	3,122	23,491
Other liabilities	5,967	32,800
Total liabilities held-for-sale	9,089	56,291
Continuing Operations [Member]
Held for Sale Assets in Continuing and Discontinued Operations
Land and Land Improvements Held-for-Sale	10,522	3,631
Buildings and Tenant Improvements Held-for-Sale	17,351	37,495
Inventory, Land Held-for-sale	17,082	22,657
Accumulated Depreciation of Assets Held-for-Sale	(6,198)	(18,581)
Lease Related Assets, Held for Sale	862	3,091
Other assets	738	3,334
Total assets held-for-sale	40,357	51,627
Accrued expenses	647	1,363
Other liabilities	802	298
Total liabilities held-for-sale	1,449	1,661
Discontinued Operations [Member]
Held for Sale Assets in Continuing and Discontinued Operations
Land and Land Improvements Held-for-Sale	26,560	118,882
Buildings and Tenant Improvements Held-for-Sale	166,573	1,218,468
Inventory, Land Held-for-sale	0	0
Accumulated Depreciation of Assets Held-for-Sale	(37,577)	(240,685)
Lease Related Assets, Held for Sale	6,882	83,522
Other assets	10,859	92,444
Total assets held-for-sale	173,297	1,272,631
Accrued expenses	2,475	22,128
Other liabilities	5,165	32,502
Total liabilities held-for-sale	$ 7,640	$ 54,630