Investments in real estate Real Estate Asset Sales (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Dispositions
Gain (loss on) sale of land parcels	$ 772,000	$ 0	$ 772,000	$ 1,864,000
Non-income producing properties
Dispositions
Gain (loss on) sale of land parcels			(121,000)
Building
Dispositions
Number of real estate properties sold			7
Aggregate consideration received			128,600,000
Building | Suburban Washington, D.C. market
Dispositions
Number of real estate properties sold			2
Gain (loss on) sale of land parcels			52,000
Building | Greater Boston market
Dispositions
Number of real estate properties sold			3
Gain (loss on) sale of land parcels			(392,000)
Building | Seattle market
Dispositions
Gain (loss on) sale of land parcels			0
Building | Property Outside of Core Markets
Dispositions
Number of real estate properties sold			1
Gain (loss on) sale of land parcels			219,000
Land
Dispositions
Number of real estate properties sold			3
Aggregate consideration received			18,100,000
Gain (loss on) sale of land parcels	772,000
Land | San Franciso Bay Area
Dispositions
Number of real estate properties sold			2
Gain (loss on) sale of land parcels	381,000
Land | New York City
Dispositions
Number of real estate properties sold			1
Gain (loss on) sale of land parcels	$ 391,000