Subsequent events (Details) $ in Thousands		1 Months Ended	3 Months Ended	9 Months Ended
		Oct. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($) ft²	Sep. 30, 2014 USD ($)
Subsequent Event [Line Items]
Repayments of borrowings from secured notes payable				$ 12,217	$ 228,909
Effective rate (as a percent)	[1]			2.97%
Area of Real Estate Property | ft²				19,943,739
Construction Loans [Member]
Subsequent Event [Line Items]
Remaining Commitment				$ 121,797
Secured notes payable
Subsequent Event [Line Items]
Effective rate (as a percent)	[1]			4.23%
1500 Owens Street [Member]
Subsequent Event [Line Items]
Area of Real Estate Property | ft²				158,267
Scenario, Forecast [Member] | 1500 Owens Street [Member]
Subsequent Event [Line Items]
Percent of ownership interest sold			49.90%
Sales price			$ 73,400
Scenario, Forecast [Member] | Number 75 and 125 Shoreway Road [Member]
Subsequent Event [Line Items]
Sales price			$ 38,500
Subsequent Event [Member] | Secured notes payable
Subsequent Event [Line Items]
Repayments of borrowings from secured notes payable		$ 76,000
Effective rate (as a percent)		5.73%
Subsequent Event [Member] | Greater Boston market | Construction Loans [Member] | Secured Debt Maturing on 26 January 2019 [Member]
Subsequent Event [Line Items]
Remaining Commitment		$ 350,000
Subsequent Event [Member] | LIBOR | Greater Boston market | Construction Loans [Member] | Secured Debt Maturing on 26 January 2019 [Member]
Subsequent Event [Line Items]
Base rate		LIBOR
Applicable margin (as a percent)		1.50%

[1] Represents the weighted-average interest rate as of the end of the period plus the impact of debt premiums/discounts and our interest rate swap agreements. The weighted-average interest rate excludes bank fees and amortization of loan fees.