Real Estate Properties (Details 3) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Income Statements:
Rental income	$ 238,790	$ 193,059	$ 662,596	$ 572,205
Operating expenses	(100,912)	(83,023)	(275,760)	(240,280)
Depreciation and amortization	(56,389)	(42,794)	(159,072)	(130,560)
General and administrative	(11,450)	(9,704)	(33,559)	(28,081)
Acquisition related costs	(4,805)	(1,559)	(9,722)	(2,965)
Operating income	65,234	55,979	184,483	148,828
Interest and other income	369	571	1,428	2,134
Interest expense	(49,423)	(44,192)	(145,037)	(133,716)
Income from discontinued operations	653	6,673	2,777	16,877
Discontinued operations, properties sold or held for sale
Income Statements:
Rental income	8,296	24,999	24,994	73,590
Operating expenses	(5,832)	(11,012)	(16,219)	(32,554)
Depreciation and amortization	(1,336)	(5,768)	(4,467)	(17,440)
General and administrative	(470)	(996)	(1,433)	(2,925)
Acquisition related costs	(5)		(148)	(7)
Operating income	653	7,223	2,727	20,664
Interest and other income		1	50	3
Interest expense		(551)		(3,790)
Income from discontinued operations	$ 653	$ 6,673	$ 2,777	$ 16,877