Investments in real estate Acquisition (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) a ft² Lease
Real Estate [Line Items]
Purchase price | $	$ 320,500
Alexandria PARC
Real Estate [Line Items]
Area of Real Estate Property	197,498
Area of Land | a	11
Purchase price | $	$ 136,000
Weighted average lease term	3 years
Summers Ridge Science Park
Real Estate [Line Items]
Area of Real Estate Property	316,531
Purchase price | $	$ 148,650
Tenant lease term	15 years
Operating property
Real Estate [Line Items]
Area of Real Estate Property	490,659
Development and redevelopment projects
Real Estate [Line Items]
Area of Real Estate Property	697,066
Redevelopment property | Alexandria PARC
Real Estate [Line Items]
Area of Real Estate Property	45,115
Future development projects
Real Estate [Line Items]
Area of Real Estate Property	50,000
Future development projects | Summers Ridge Science Park
Real Estate [Line Items]
Area of Land	50,000
Equity Method Investee | 1655 and 1725 Third Street
Real Estate [Line Items]
Area of Real Estate Property	593,765
Payments to Acquire Interest in Joint Venture | $	$ 31,950
Equity interest percentage (in percent)	10.00%
Equity Method Investee | 704 Quince Orchard Road
Real Estate [Line Items]
Area of Real Estate Property	79,931
Payments to Acquire Interest in Joint Venture | $	$ 3,900
Equity interest percentage (in percent)	56.80%
Equity Method Investee | 704 Quince Orchard Road | Redevelopment property
Real Estate [Line Items]
Area of Real Estate Property	58,186
Leases, Acquired-in-Place | Alexandria PARC
Real Estate [Line Items]
Number of Leases Held | Lease	14