Consolidated and unconsolidated real estate joint ventures (Details)	Mar. 31, 2018 ft²
225 Binney Street
Schedule of Equity Method Investments [Line Items]
Equity interest percentage (in percent)	30.00%
Area of Real Estate Property	305,212
409/499 Illinois Street
Schedule of Equity Method Investments [Line Items]
Equity interest percentage (in percent)	60.00%
Area of Real Estate Property	455,069
1500 Owens Street
Schedule of Equity Method Investments [Line Items]
Equity interest percentage (in percent)	50.10%
Area of Real Estate Property	158,267
Campus Pointe by Alexandria
Schedule of Equity Method Investments [Line Items]
Equity interest percentage (in percent)	55.00%
Area of Real Estate Property	798,799
9625 Towne Centre Drive
Schedule of Equity Method Investments [Line Items]
Area of Real Estate Property	163,648
Equity Method Investee | Menlo Gateway
Schedule of Equity Method Investments [Line Items]
Area of Real Estate Property	772,983
Equity Method Investee | 1401/1413 Research Boulevard
Schedule of Equity Method Investments [Line Items]
Equity interest percentage (in percent)	65.00%
Area of Real Estate Property	90,000
Equity Method Investee | 360 Longwood Avenue
Schedule of Equity Method Investments [Line Items]
Equity interest percentage (in percent)	27.50%
Area of Real Estate Property	210,709
Equity Method Investee | 704 Quince Orchard Road
Schedule of Equity Method Investments [Line Items]
Equity interest percentage (in percent)	56.80%
Area of Real Estate Property	79,931
Equity Method Investee | 1655 and 1725 Third Street
Schedule of Equity Method Investments [Line Items]
Equity interest percentage (in percent)	10.00%
Area of Real Estate Property	593,765
Initial ownership interest | 9625 Towne Centre Drive
Schedule of Equity Method Investments [Line Items]
Equity interest percentage (in percent)	54.70%
Initial ownership interest | Equity Method Investee | Menlo Gateway
Schedule of Equity Method Investments [Line Items]
Equity interest percentage (in percent)	25.20%