Discontinued operations (Details) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2012 property	Sep. 30, 2011 property	Jun. 30, 2011	Jun. 30, 2012 property	Jun. 30, 2011	Dec. 31, 2011
Net assets of discontinued operations
Investment in real estate "held for sale," net	$ 18,851,000			$ 18,851,000		$ 20,851,000
Other assets	695,000			695,000		847,000
Total assets	19,546,000			19,546,000		21,698,000
Total liabilities	(139,000)			(139,000)		(469,000)
Net assets of discontinued operations	19,407,000			19,407,000		21,229,000
(Loss) income from discontinued operations, net
Total revenues	318,000		657,000	602,000	1,276,000
Operating expenses	75,000		126,000	185,000	237,000
Total revenues less operating expenses	243,000		531,000	417,000	1,039,000
Interest expense			4,000		36,000
Depreciation expense	39,000		190,000	78,000	354,000
Income from discontinued operations before gain on sales of real estate	204,000		337,000	339,000	649,000
Gain on sale of real estate	2,000			2,000
Income from discontinued operations, net	206,000		337,000	341,000	649,000
Number of operating properties classified as held for sale				3
Number of properties sold	1	1
5110 Campus Drive
Dispositions
Disposition Amount	1,800,000			1,800,000
Gain on sale of properties	$ 2,000			$ 2,000