Investments in Unconsolidated Companies (Summarized Financial Information) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity Method Investments and Joint Ventures [Abstract]
Rental revenue	$ 71,424	$ 122,019	$ 160,543
Equity Method Investment, Gain (Loss) on Sale of Properties	4,986	100,806	23,696
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	20,673	122,727	60,772
Equity in earnings (loss) of unconsolidated joint ventures	63,310	47,403	$ (3,304)
Land, buildings and tenant improvements, net	383,581	529,926
Construction in progress	65,715	31,838
Undeveloped land	30,170	90,560
Other assets	76,695	91,045
Total assets	556,161	743,369
Indebtedness	235,497	172,811
Other liabilities	39,497	32,633
Total liabilities	274,994	205,444
Owners' equity	281,167	537,925
Total liabilities and equity	556,161	743,369
Investments in and advances to unconsolidated joint ventures	126,487	197,807
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	$ 6,400	$ 22,200