Discontinued Operations and Assets Held for Sale (Table Illustration of Discontinued Operations in Income Statement) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Discontinued Operations
Payments For Second Generation Tenant Improvements Leasing Costs, Building Improvements and development costs	$ 52,554	$ 59,349	$ 61,900
Revenues	87,185	172,716	189,805
Operating expenses	(28,102)	(52,795)	(61,916)
Depreciation and Amortization, Discontinued Operations	(25,911)	(75,261)	(75,082)
Operating income	33,172	44,660	52,807
Interest expense	(14,736)	(28,819)	(40,911)
Income before gain on sales	18,436	15,841	11,896
Gain on sale of depreciable properties	1,357,778	1,016	424,892
Income from discontinued operations before income taxes	1,376,214	16,857	436,788
Income tax expense	(12,465)	0	(3,175)
Income from discontinued operations	1,363,749	16,857	433,613
Discontinued Operations [Member]
Discontinued Operations
Payments For Second Generation Tenant Improvements Leasing Costs, Building Improvements and development costs	$ 20,900	$ 33,700	$ 131,300