Investments in real estate (Details 4) (USD $)	3 Months Ended	6 Months Ended		1 Months Ended
	Jun. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Apr. 30, 2012 Longwood Medical Area item sqft Y	Mar. 31, 2012 Longwood Medical Area sqft Y	Jun. 30, 2012 Longwood Medical Area
Investment in unconsolidated real estate entity
Ownership interest in land parcel before sale (as a percent)					55.00%
Ownership interest in land parcel after sale (as a percent)					27.50%
Ownership interest of admitted member in real estate joint venture (as a percent)					50.00%
Developable square feet of land parcel					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
Recourse secured debt proceeds					5,400,000
Existing recourse secured loan					38,400,000
Secured construction loan					213,000,000
Term of construction loan					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
Outstanding balance of construction loan						51,100,000
Capital contributions through the completion of the project					22,300,000
Project cost				350,000,000
Percentage of project pre-leased				37.00%
Number of additional floors available for occupancy to Dana-Farber Cancer Institute				2
Square feet of land available for occupancy to Dana-Farber Cancer Institute				99,000
Square feet of land available for occupancy to Dana-Farber Cancer Institute (as a percent)				24.00%
Development and other fees receivable				3,500,000
Development fees	200,000	200,000
Investment in unconsolidated real estate entity	$ 26,874,000	$ 26,874,000	$ 42,342,000