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, 2018 USD ($) a buildings	Dec. 31, 2017 USD ($)
Held for Sale Assets in Continuing and Discontinued Operations
Continuing Operations, Number of In-service Properties Held for Sale | buildings	1
Number of Real Estate Properties Held for Sale | buildings	1
Land and Land Improvements Held-for-Sale | a	18
Total assets held-for-sale	$ 53,653	$ 17,550
Total liabilities held-for-sale	606	1,163
Continuing Operations [Member]
Held for Sale Assets in Continuing and Discontinued Operations
Land and Land Improvements Held-for-Sale	10,158	8,157
Buildings and Tenant Improvements Held-for-Sale	51,567	10,505
Accumulated Depreciation of Assets Held-for-Sale	(9,982)	(2,553)
Deferred leasing and other costs, net	886	862
Other assets	1,024	579
Total assets held-for-sale	53,653	17,550
Total liabilities held-for-sale	$ 606	$ 1,163