Investment in unconsolidated real estate joint ventures (Details) $ in Thousands	6 Months Ended
Jun. 30, 2016 USD ($) ft²
Construction Loans \| Longwood [Member] \| Secured Debt from Bank Maturing on 1 April 2017 [Member]
Schedule of Equity Method Investments [Line Items]
Maturity Date	Apr. 01, 2017
London Interbank Offered Rate (LIBOR) [Member] \| Construction Loans \| Longwood [Member] \| Secured Debt from Bank Maturing on 1 April 2017 [Member]
Schedule of Equity Method Investments [Line Items]
Applicable margin (as a percent)	3.75%
Greater Boston market \| Longwood [Member]
Schedule of Equity Method Investments [Line Items]
Area of Real Estate Property \| ft²	413,799
Expected Total Joint Venture Development Cost	$ 350,000
Rentable Square Footage of Real Estate Property, Operating Properties \| ft²	313,407
Real estate occupancy percentage	76.00%
Equity Method Investments	$ 50,000
Greater Boston market \| Construction Loans \| Longwood [Member] \| Secured Debt from Bank Maturing on 1 April 2017 [Member]
Schedule of Equity Method Investments [Line Items]
Maturity Date	Apr. 01, 2017
Stated interest rate (as a percent)	5.25%
Greater Boston market \| Construction Loan, Aggregate Commitments \| Longwood [Member] \| Secured Debt from Bank Maturing on 1 April 2017 [Member]
Schedule of Equity Method Investments [Line Items]
Long-term Construction Loan	$ 213,200
Long-term debt at fixed interest rate	175,241
Long-term debt at variable interest rate	37,959
Greater Boston market \| Construction Loan, Outstanding Balance \| Longwood [Member] \| Secured Debt from Bank Maturing on 1 April 2017 [Member]
Schedule of Equity Method Investments [Line Items]
Long-term Construction Loan	180,693
Unamortized debt issuance expense	352
Long-term debt at fixed interest rate	173,226
Long-term debt at variable interest rate	7,467
Long-term Construction Loan, Net of Unamortized Deferred Financing Costs	$ 180,341
Greater Boston market \| Secured Debt from Bank Maturing on 1 April 2017 [Member] \| Longwood [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Number of One-Year Maturity Date Extension Option	2
Debt Instrument, Extended Maturity Period	1 year
Greater Boston market \| Construction Loan, Remaining Commitments [Member] \| Longwood [Member] \| Secured Debt from Bank Maturing on 1 April 2017 [Member]
Schedule of Equity Method Investments [Line Items]
Long-term Construction Loan	$ 32,507
Long-term Debt, Percentage Bearing Fixed Interest, Remaining Commitments	2,015
Long-term Debt, Percentage Bearing Variable Interest, Remaining Commitments	$ 30,492
San Francisco Bay Area \| 1455/1515 Third Street
Schedule of Equity Method Investments [Line Items]
Area of Real Estate Property \| ft²	422,980
Real estate occupancy percentage	100.00%
Length of Lease	15 years
Equity interest percentage (in percent)	51.00%
Equity Method Investments	$ 82,500
Equity Method Investee \| Greater Boston market \| Longwood [Member]
Schedule of Equity Method Investments [Line Items]
Equity interest percentage (in percent)	27.50%
Uber Technologies, Inc. \| San Francisco Bay Area \| 1455/1515 Third Street
Schedule of Equity Method Investments [Line Items]
Equity interest percentage (in percent)	49.00%
Minimum \| Construction Loans \| Longwood [Member] \| Secured Debt from Bank Maturing on 1 April 2017 [Member]
Schedule of Equity Method Investments [Line Items]
Stated interest rate (as a percent)	5.25%
Maximum \| London Interbank Offered Rate (LIBOR) [Member] \| Construction Loans \| Longwood [Member] \| Secured Debt from Bank Maturing on 1 April 2017 [Member]
Schedule of Equity Method Investments [Line Items]
Applicable margin (as a percent)	3.50%

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