Discontinued Operations and Assets Held for Sale (Table Illustration of Discontinued Operations in Income Statement) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Discontinued Operations
Payments For Second Generation Tenant Improvements Leasing Costs, Building Improvements and development costs	$ 53,474	$ 52,554	$ 59,349
Revenues	117	87,185	172,716
Operating expenses	(9)	(28,102)	(52,795)
Depreciation and Amortization, Discontinued Operations	0	(25,911)	(75,261)
Operating income	108	33,172	44,660
Interest expense	0	(14,736)	(28,819)
Income before gain on sales and income taxes	108	18,436	15,841
Gain on sale of depreciable properties	3,792	1,357,778	1,016
Income from discontinued operations before income taxes	3,900	1,376,214	16,857
Income tax expense	0	(12,465)	0
Income from discontinued operations	$ 3,900	1,363,749	16,857
Discontinued Operations [Member]
Discontinued Operations
Payments For Second Generation Tenant Improvements Leasing Costs, Building Improvements and development costs		$ 20,900	$ 33,700