Segment Information (Reconciliation of Consolidated Net Operating Income) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 Reportable_Segments	Jun. 30, 2013
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Consolidated net operating income	$ 92,793	$ 85,671	$ 182,933	$ 170,156
Other income (expense):
Depreciation and amortization	(52,587)	(49,241)	(105,157)	(98,717)
General and administrative expenses	(6,005)	(7,336)	(14,186)	(13,887)
Interest income	385	122	770	180
Interest expense	(31,512)	(30,437)	(63,356)	(61,351)
Amortization of deferred financing costs	(1,197)	(1,183)	(2,386)	(2,344)
Interest expense — financing obligation	(316)	(211)	(588)	(429)
Equity in income (loss) of real estate ventures	(489)	1,508	(247)	3,043
Gain (Loss) on sale of undepreciated real estate	(3)	0	1,184	0
Gain from remeasurement of investment in real estate ventures	458	7,847	458	7,847
Gain (loss) on real estate venture transactions	(282)	3,683	(417)	3,683
Loss on early extinguishment of debt	0	(1,113)	0	(1,116)
Income (loss) from continuing operations	1,245	9,310	(992)	7,065
Income (Loss) from Discontinued Operations	929	(2,130)	921	4,034
Net income	$ 2,174	$ 7,180	$ (71)	$ 11,099
Number of Reportable Segments			7