Consolidated and unconsolidated real estate joint ventures (Details)	3 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) ft²	Mar. 31, 2019 USD ($) ft²	Dec. 31, 2019 USD ($) ft²	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Equity Method Investments
Contributions from and sales of noncontrolling interests			$ 1,134,939,000	$ 27,418,000	$ 52,625,000
225 Binney Street
Schedule of Equity Method Investments
Equity interest percentage (in percent)			30.00%
75/125 Binney Street
Schedule of Equity Method Investments
Equity interest percentage (in percent)			40.00%
Noncontrolling interest share (in percentage)		60.00%
Area of Real Estate Property | ft²		388,270
Proceeds from Sale of Real Estate		$ 438,000,000.0
Proceeds from Sale of Real Estate (Per RSF)		1,880
409/499 Illinois Street
Schedule of Equity Method Investments
Equity interest percentage (in percent)			60.00%
1500 Owens Street
Schedule of Equity Method Investments
Equity interest percentage (in percent)			50.10%
500 Forbes Boulevard
Schedule of Equity Method Investments
Equity interest percentage (in percent)			10.00%
Noncontrolling interest share (in percentage)	90.00%
Area of Real Estate Property | ft²	155,685
Proceeds from Sale of Real Estate	$ 139,500,000
Proceeds from Sale of Real Estate (Per RSF)	$ 996
Campus Pointe by Alexandria
Schedule of Equity Method Investments
Equity interest percentage (in percent)			55.00%
5200 Illumina Way
Schedule of Equity Method Investments
Equity interest percentage (in percent)			51.00%
Noncontrolling interest share (in percentage)	49.00%
Number of Real Estate Properties	6
Area of Real Estate Property | ft²	792,687
Proceeds from Sale of Real Estate	$ 585,200,000
9625 Towne Centre Drive
Schedule of Equity Method Investments
Equity interest percentage (in percent)			50.10%
SD Tech by Alexandria
Schedule of Equity Method Investments
Equity interest percentage (in percent)			50.00%
Equity Method Investee | Menlo Gateway
Schedule of Equity Method Investments
Equity interest percentage (in percent)			49.00%
Equity Method Investee | 1401/1413 Research Boulevard
Schedule of Equity Method Investments
Equity interest percentage (in percent)			65.00%
Equity Method Investee | 704 Quince Orchard Road
Schedule of Equity Method Investments
Equity interest percentage (in percent)			56.80%
Equity Method Investee | 1655 and 1715 Third Street
Schedule of Equity Method Investments
Equity interest percentage (in percent)			10.00%
Additional Paid-In Capital
Schedule of Equity Method Investments
Contributions from and sales of noncontrolling interests			$ 381,162,000	257,000	7,747,000
Additional Paid-In Capital | 75/125 Binney Street
Schedule of Equity Method Investments
Contributions from and sales of noncontrolling interests		$ 202,200,000
Additional Paid-In Capital | 500 Forbes Boulevard
Schedule of Equity Method Investments
Contributions from and sales of noncontrolling interests	48,400,000
Additional Paid-In Capital | 5200 Illumina Way
Schedule of Equity Method Investments
Contributions from and sales of noncontrolling interests	269,100,000
Noncontrolling Interests
Schedule of Equity Method Investments
Contributions from and sales of noncontrolling interests			$ 753,777,000	$ 27,161,000	$ 44,878,000
Noncontrolling Interests | 5200 Illumina Way
Schedule of Equity Method Investments
Proceeds from Sale of Real Estate	286,700,000
Contributions from and sales of noncontrolling interests	131,900,000
Operating property
Schedule of Equity Method Investments
Area of Real Estate Property | ft²			1,949,100
Operating property | Noncontrolling Interests | 5200 Illumina Way
Schedule of Equity Method Investments
Proceeds from Sale of Real Estate	264,600,000
Proceeds from Sale of Real Estate (Per RSF)	681
Future development
Schedule of Equity Method Investments
Area of Real Estate Property | ft²			2,350,338
Future development | Noncontrolling Interests | 5200 Illumina Way
Schedule of Equity Method Investments
Proceeds from Sale of Real Estate	22,100,000
Proceeds from Sale of Real Estate (Per RSF)	$ 100