Investments In and Advances To Unconsolidated Affiliates (Details) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2016 USD ($)	Mar. 31, 2016 USD ($) property	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Income Statements:
Rental and other revenues	$ 10,357		$ 12,323	$ 32,339	$ 36,977
Expenses:
Rental property and other expenses	4,201		5,985	13,489	17,683
Depreciation and amortization	2,459		3,193	7,862	9,418
Interest expense	1,264		1,645	3,983	5,826
Total expenses	7,924		10,823	25,334	32,927
Income before disposition of property	2,433		1,500	7,005	4,050
Gains/(losses) on disposition of property	21,345		0	22,247	18,181
Net income	$ 23,778		$ 1,500	$ 29,252	$ 22,231
Percentage of equity interest in joint ventures, maximum (in hundredths)	50.00%			50.00%
Concourse Center Associates, LLC Joint Venture [Member]
Expenses:
Gains/(losses) on disposition of property		$ (100)
Percentage of equity interest in joint ventures, maximum (in hundredths)		50.00%
Proceeds received from disposition of property		$ 11,000
Our share of gains on disposition of property		400
Repayment of debt		$ 6,600
Number of buildings sold | property		2
4600 Madison Associates, LLC Joint Venture [Member]
Expenses:
Gains/(losses) on disposition of property	$ 21,300	$ 1,000
Percentage of equity interest in joint ventures, maximum (in hundredths)	12.50%			12.50%
Proceeds received from disposition of property	$ 32,700	3,400
Our share of gains on disposition of property	1,800	100
Repayment of debt	$ 6,300	$ 3,400		$ 6,300