Investments in real estate (Details 4) (USD $)	3 Months Ended	12 Months Ended	1 Months Ended
	Mar. 31, 2012 sqft	Dec. 31, 2011 sqft	Mar. 31, 2012 Longwood Medical Area sqft Y
Investment in unconsolidated real estate entity
Investment in unconsolidated real estate entity	$ 25,870,000	$ 42,342,000
Ownership interest in land parcel before sale (as a percent)			55.00%
Ownership interest in land parcel after sale (as a percent)			55.00%
Ownership interest of admitted member in real estate joint venture (as a percent)			50.00%
Developable square feet of land parcel	11,662,000	10,939,000	414,000
Ownership interest in land parcel sold (as a percent)			27.50%
Special distribution from sale of land parcel	22,250,000		22,300,000
Gain on sale of land parcel	(1,864,000)		(1,900,000)
Non-recourse secured debt proceeds			5,400,000
Existing non-recourse secured loan			38,400,000
Construction loan			213,000,000
Term of construction loan (in years)			7 years
Number of extension options (in years)			2
Period of extension options (in years)			1
Initial loan proceeds from construction loan			50,000,000
Capital contributions through the completion of the project			22,300,000
Project cost			350,000,000
Percentage of project pre-leased			37.00%
Development and other fees receivable			$ 3,500,000