Discontinued operations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013 property		Dec. 31, 2012		Dec. 31, 2011 property
Net assets of discontinued operations
Properties "held for sale, "net	$ 7,644		$ 133,111
Other assets	103		1,714
Total assets	7,747		134,825
Total liabilities	(266)		(5,063)
Net assets of discontinued operations	7,481		129,762
Income from discontinued operations, net
Total revenues	4,657		36,283		37,744
Operating expenses	(1,981)		(11,313)		(11,482)
Total revenues less operating expenses from discontinued operations	2,676		24,970		26,262
Interest expense	0		0		(71)
Depreciation expense	(1,655)		(6,318)		(7,120)
Loss (gain) on sale of real estate	(121)		1,564		0
Impairment of real estate	0		(11,400)		(994)
Income from discontinued operations	$ 900	[1]	$ 8,816	[1]	$ 18,077	[1]
Number of operating properties classified as held for sale	4
Number of properties sold					14

[1] Income from discontinued operations includes the results of operations of four properties that were classified as “held for sale” as of December 31, 2013, as well as the results of operations (prior to disposition) and gain/loss on sale of real estate attributable to 14 properties sold during the period from January 1, 2011, to December 31, 2013.