Real Estate Properties (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Statements of Operations:
Rental income	$ 274,766	$ 249,797	$ 549,814	$ 493,175
Operating expenses	(109,754)	(103,034)	(219,413)	(200,270)
Depreciation and amortization	(67,389)	(60,433)	(133,912)	(118,452)
General and administrative	(21,653)	(12,595)	(38,919)	(24,131)
Operating income	75,825	72,301	156,797	146,386
Interest and other income	250	360	708	645
Loss from discontinued operations	(311)	(3,317)	(2,223)	(6,406)
Discontinued operations, properties sold or held for sale
Statements of Operations:
Rental income	4,625	5,577	9,432	13,445
Operating expenses	(4,379)	(5,059)	(10,414)	(11,913)
Depreciation and amortization		(3,119)		(6,451)
General and administrative	(565)	(769)	(1,249)	(1,543)
Operating income	(319)	(3,370)	(2,231)	(6,462)
Interest and other income	8	53	8	56
Loss from discontinued operations	$ (311)	$ (3,317)	$ (2,223)	$ (6,406)