Investments in real estate, net Sales of land parcels (Details) (USD $)	12 Months Ended					1 Months Ended		3 Months Ended	12 Months Ended			12 Months Ended
	Dec. 31, 2013 property	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Land Parcel	Dec. 31, 2013 San Francisco Bay Area Land Parcel	May 31, 2013 New Jersey Land	Dec. 31, 2011 New Jersey Land	Dec. 31, 2012 New Jersey Land sqft	Dec. 31, 2013 New Jersey Land Parcel sqft	Dec. 31, 2013 Greater Boston market Equity Method Investee sqft	Dec. 31, 2012 Greater Boston market Equity Method Investee	Dec. 31, 2013 Clarion Partners, LLC Greater Boston market Equity Method Investee
Dispositions
Number of real estate properties sold	7			4	3				1
Aggregate consideration received				$ 73,300,000
Area of real estate property								50,000		413,536
Gain (loss on) sale of properties	4,824,000	1,864,000	46,000	4,824,000	4,432,000	391,000	46,000		391,000			1,900,000
Percent of ownership interest in joint venture sold												50.00%
Equity interest percentage (in percent)										27.50%	55.00%
Distribution from unconsolidated real estate entity	0	22,250,000	0									22,300,000
Share of proceeds from loan refinancing										5,400,000
Impairment of real estate	0	11,400,000	994,000					2,100,000
Proceeds from sale of real estate	$ 128,600,000						$ 17,300,000	$ 2,000,000