Secured and unsecured senior debt Schedule of secured construction loans (Details) - Construction Loans [Member]		1 Months Ended	6 Months Ended	9 Months Ended
		Oct. 31, 2015 USD ($)	Jun. 30, 2015 Extension_Option	Sep. 30, 2015 USD ($) Extension_Option
Debt Instrument [Line Items]
Outstanding Balance				$ 219,603,000
Remaining Commitment				121,797,000
Total Aggregate Commitments				$ 341,400,000
San Franciso Bay Area [Member] | Secured Debt Maturing on 1 June 2016 [Member]
Debt Instrument [Line Items]
Maturity Date	[1]			Jun. 01, 2016
Outstanding Balance				$ 20,714,000
Remaining Commitment				15,286,000
Total Aggregate Commitments				$ 36,000,000
Number of extensions available under line of credit | Extension_Option				2
Term of debt extension				1 year
San Franciso Bay Area [Member] | Secured Debt from Bank Maturing on 1 July 2016 [Member]
Debt Instrument [Line Items]
Maturity Date	[2]			Jul. 01, 2016
Outstanding Balance				$ 47,385,000
Remaining Commitment				7,615,000
Total Aggregate Commitments				$ 55,000,000
Number of extensions available under line of credit | Extension_Option			2
Term of debt extension			1 year	1 year
Greater Boston market | Secured Debt Maturing on 23 August 2017 [Member]
Debt Instrument [Line Items]
Maturity Date	[3]			Aug. 23, 2017
Outstanding Balance				$ 151,504,000
Remaining Commitment				98,896,000
Total Aggregate Commitments				$ 250,400,000
Term of debt extension				1 year
Greater Boston market | Secured Debt Maturing on 26 January 2019 [Member] | Subsequent Event [Member]
Debt Instrument [Line Items]
Outstanding Balance		$ 0
Remaining Commitment		350,000,000
Total Aggregate Commitments		$ 350,000,000
LIBOR | San Franciso Bay Area [Member] | Secured Debt Maturing on 1 June 2016 [Member]
Debt Instrument [Line Items]
Base rate				LIBOR
Applicable margin (as a percent)				1.40%
LIBOR | San Franciso Bay Area [Member] | Secured Debt from Bank Maturing on 1 July 2016 [Member]
Debt Instrument [Line Items]
Base rate				LIBOR
Applicable margin (as a percent)				1.50%
LIBOR | Greater Boston market | Secured Debt Maturing on 23 August 2017 [Member]
Debt Instrument [Line Items]
Base rate				LIBOR
Applicable margin (as a percent)				1.35%
LIBOR | Greater Boston market | Secured Debt Maturing on 26 January 2019 [Member] | Subsequent Event [Member]
Debt Instrument [Line Items]
Base rate		LIBOR
Applicable margin (as a percent)		1.50%
Longwood [Member] | LIBOR
Debt Instrument [Line Items]
Applicable margin (as a percent)				3.75%

[1] We have two, one-year options to extend the stated maturity date to June 1, 2018, subject to certain conditions.

[2] We have a one-year option to extend the stated maturity date to July 1, 2017, subject to certain conditions.

[3] We have a one-year option to extend the stated maturity date to August 23, 2018, subject to certain conditions.During October 2015, we executed the following secured construction loan (dollars in thousands):Market Stated Rate Maturity Date Outstanding Balance Remaining Commitments Total CommitmentsGreater Boston (1) L+1.50% 1/28/19 $— $350,000 $350,000(1)In October 2015, closed a secured construction loan with aggregate commitments available for borrowing of $350.0 million, for our 98% leased development project at 50/60 Binney Street in our Cambridge submarket, which bears interest at a rate of LIBOR+150 bps.