Secured and unsecured senior debt Schedule of secured construction loans (Details) (Construction Loans, USD $)	6 Months Ended
	Jun. 30, 2014
Debt Instrument [Line Items]
Outstanding Balance	$ 123,775,000
Remaining Commitment	217,625,000
Total Aggregate Commitments	341,400,000
San Francisco Bay Area | Secured notes payable maturing on 7/1/15
Debt Instrument [Line Items]
Maturity Date	Jul. 01, 2015	[1]
Outstanding Balance	46,399,000
Remaining Commitment	8,601,000
Total Aggregate Commitments	55,000,000
San Francisco Bay Area | Secured notes payable maturing on 06/1/16
Debt Instrument [Line Items]
Maturity Date	Jun. 01, 2016	[2]
Outstanding Balance	11,936,000
Remaining Commitment	24,064,000
Total Aggregate Commitments	36,000,000
Greater Boston market | Secured notes payable maturing on 8/23/17
Debt Instrument [Line Items]
Maturity Date	Aug. 23, 2017	[3]
Outstanding Balance	65,440,000
Remaining Commitment	184,960,000
Total Aggregate Commitments	$ 250,400,000
LIBOR | San Francisco Bay Area | Secured notes payable maturing on 7/1/15
Debt Instrument [Line Items]
Base rate	LIBOR
Applicable margin (as a percent)	1.50%
LIBOR | San Francisco Bay Area | Secured notes payable maturing on 06/1/16
Debt Instrument [Line Items]
Base rate	LIBOR
Applicable margin (as a percent)	1.40%
LIBOR | Greater Boston market | Secured notes payable maturing on 8/23/17
Debt Instrument [Line Items]
Base rate	LIBOR
Applicable margin (as a percent)	1.35%

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

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