Investments in real estate, net Real estate asset sales (Details) (USD $)	12 Months Ended			1 Months Ended	3 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended		12 Months Ended
	Dec. 31, 2013 property	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Number 1124 Columbia Street	Sep. 30, 2012 Number 1124 Columbia Street sqft	Dec. 31, 2012 Number 1124 Columbia Street	Sep. 30, 2012 One Innovation Drive, 377 and 381 Plantation Street sqft	Sep. 30, 2012 One Innovation Drive, 377 and 381 Plantation Street sqft	Dec. 31, 2012 One Innovation Drive, 377 and 381 Plantation Street	Dec. 31, 2011 Greater Boston market	Sep. 30, 2011 Greater Boston market sqft	Dec. 31, 2013 1124 Columbia Street, 23/35/45 West Watkins Mill Road, 1201 Clopper Road, One Innovation Drive, 377 and 381 Plantation Street, and 702 Electronic
Real Estate Properties [Line Items]
Number of real estate properties sold	7
Proceeds from sale of real estate	$ 128,600,000			$ 40,600,000			$ 39,600,000			$ 2,900,000
Gain (loss) on disposal of properties recognized	121,000
Area of real estate property					203,817		300,313	300,313			30,000
Average percentage of occupied properties					97.00%			92.00%
Annual operating income generated by property	2,676,000	24,970,000	26,262,000			6,200,000			6,600,000
Impairment of real estate	0	11,400,000	994,000	1,600,000	4,800,000			5,000,000			1,000,000
Selling price of disposal real estate				42,600,000		42,600,000
Gain (loss on) sale of properties	$ 4,824,000	$ 1,864,000	$ 46,000									$ (121,000)