Discontinued operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013 property		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2012
Net assets of discontinued operations
Properties "held for sale, "net	$ 4,510				$ 4,510				$ 76,440
Other assets	14				14				4,546
Total assets	4,524				4,524				80,986
Total liabilities	(32)				(32)				(3,233)
Net assets of discontinued operations	4,492				4,492				77,753
Income from discontinued operations, net
Total revenues	4		8,418		3,741		26,556
Operating expenses	68		2,788		1,697		8,337
Total revenues less operating expenses from discontinued operations	(64)		5,630		2,044		18,219
Depreciation expense	0		1,589		930		4,822
Loss (gain) on sale of real estate	0		(1,562)		121		(1,564)
Impairment of real estate	0		9,799		0		9,799
(Loss) income from discontinued operations, net	$ (64)	[1]	$ (4,196)	[1]	$ 993	[1]	$ 5,162	[1]
Number of operating properties classified as held for sale	2
Number of properties sold	10

[1] (Loss) income from discontinued operations, net, includes the results of operations for two operating properties that were classified as “held for sale” as of September 30, 2013, as well as the results of operations (prior to disposition) and (gain) loss on sale of real estate attributable to 10 properties sold during the period from January 1, 2012, to September 30, 2013.