Discontinued operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2012 property	Dec. 31, 2011 property	Sep. 30, 2011	Sep. 30, 2012 property	Sep. 30, 2011
Net assets of discontinued operations
Properties "held for sale, "net	$ 77,702	$ 88,408		$ 77,702
Other assets	4,010	4,176		4,010
Total assets	81,712	92,584		81,712
Total liabilities	(2,435)	(3,532)		(2,435)
Net assets of discontinued operations	79,277	89,052		79,277
(Loss) income from discontinued operations, net
Total revenues	5,814		6,296	18,808	19,658
Operating expenses	(2,360)		(2,254)	(7,181)	(6,986)
Total revenues less operating expenses	3,454		4,042	11,627	12,672
Interest expense					(65)
Depreciation expense	(998)		(1,243)	(3,156)	(3,734)
Gain on sale of real estate	1,562			1,564
Income from discontinued operations before impairment of real estate	4,018		2,799	10,035	8,873
Impairment of real estate	(9,799)		(994)	(9,799)	(994)
(Loss) income from discontinued operations, net	$ (5,781)		$ 1,805	$ 236	$ 7,879
Number of operating properties classified as held for sale	4			4
Number of properties sold	5	1		6