Investments in real estate (Details 4) (USD $)	3 Months Ended	9 Months Ended			1 Months Ended		3 Months Ended
	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Apr. 30, 2012 Longwood Medical Area sqft item	Mar. 31, 2012 Longwood Medical Area sqft	Mar. 31, 2012 Longwood Medical Area	Sep. 30, 2012 Longwood Medical Area
Investment in unconsolidated real estate entity
Ownership interest in land parcel before sale (as a percent)						55.00%	55.00%
Ownership interest in land parcel after sale (as a percent)						27.50%	27.50%
Ownership interest of admitted member in real estate joint venture (as a percent)						50.00%	50.00%
Developable square feet of land parcel						414,000
Ownership interest in land parcel sold (as a percent)						27.50%	27.50%
Special distribution from sale of land parcel		$ 22,250,000				$ 22,300,000
Gain (loss) on sale of land parcel	46,000	1,864,000	46,000			1,900,000
Recourse secured debt proceeds						5,400,000
Existing recourse secured loan						38,400,000
Secured construction loan						213,000,000	213,000,000
Term of construction loan						7 years	7 years
Number of extension options							2 years
Period of extension options							1 year
Initial loan proceeds from construction loan						50,000,000
Outstanding balance of construction loan								56,400,000
Capital contributions through the completion of the project						22,300,000	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
Investment in unconsolidated real estate entity		$ 26,998,000		$ 42,342,000