Segment Information Segment Information (Reconciliation of Consolidated Net Operating Income) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Reportable_Segments	Jun. 30, 2012
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Consolidated net operating income	$ 85,851	$ 80,668	$ 170,494	$ 161,879
Less:
Interest expense	(30,437)	(32,981)	(61,351)	(67,125)
Interest expense — amortization of deferred financing costs	(1,183)	(1,261)	(2,344)	(2,572)
Depreciation and amortization	(49,300)	(47,476)	(98,846)	(95,350)
General and administrative expenses	7,335	6,079	13,886	12,129
Plus:
Interest income	122	1,838	180	2,320
Interest expense — financing obligation	(211)	(196)	(429)	(378)
Equity in income of real estate ventures	1,508	838	3,043	882
Gain from remeasurement of investment in a real estate venture	7,847	0	7,847	0
Net gain (loss) on real estate venture transactions	3,683		3,683	0
Gains (Losses) on Sales of Investment Real Estate		(11)		(11)
Loss on early extinguishment of debt	(1,113)	(1,250)	(1,116)	(1,498)
Income (loss) from continuing operations	9,432	(5,910)	7,275	(13,982)
Income (Loss) from Discontinued Operations	(2,252)	12,712	3,824	30,120
Net income (loss)	$ 7,180	$ 6,802	$ 11,099	$ 16,138
Number of Reportable Segments			7