Real Estate Properties (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Statements of Operations:
Rental income	$ 201,741	$ 226,802	$ 699,207	$ 662,587
Operating expenses	(94,632)	(92,005)	(287,650)	(265,227)
Depreciation and amortization	(52,150)	(52,353)	(169,930)	(154,623)
General and administrative	(25,069)	(12,565)	(61,445)	(34,190)
Operating income	30,326	68,813	179,845	203,545
Interest and other income	227	399	931	1,020
Interest expense	(39,359)	(50,736)	(134,824)	(148,924)
(Loss) income from discontinued operations	(28)	774	4,390	4,891
Discontinued operations, properties sold or held for sale
Statements of Operations:
Rental income	27,619	34,859	89,399	105,694
Operating expenses	(17,417)	(20,572)	(54,226)	(59,533)
Depreciation and amortization	(8,157)	(11,084)	(24,289)	(33,717)
General and administrative	(1,761)	(2,027)	(5,553)	(6,076)
Operating income	284	1,176	5,331	6,368
Interest and other income	1		13	80
Interest expense	(313)	(402)	(954)	(1,557)
(Loss) income from discontinued operations	$ (28)	$ 774	$ 4,390	$ 4,891