Investments in real estate (Details 2) (USD $)	6 Months Ended	12 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Investments in real estate
Rental properties, net	$ 4,117,611,000		$ 3,930,762,000
Rentable square feet of rental properties (in square feet)	12,672,852		12,500,000
Rentable square feet of land held for future development (in square feet)	12,000,000		8,300,000
Rentable square feet, development properties (in square feet)	690,139		475,818
Rentable square feet of construction projects in India and China (in square feet)	900,000		900,000
Total interest capitalized	28,200,000	41,500,000
Rentable square feet undergoing active redevelopment (in square feet)	782,258		755,463
Land held for future development	$ 534,618,000		$ 431,838,000
Rental properties, construction in progress, and land held for future development
Square feet undergoing preconstruction activities (in square feet)	2,400,000		3,000,000
Number of largest projects included in preconstruction	2
Alexandria Center at Kendall Square in East Cambridge, Massachusetts
Rental properties, construction in progress, and land held for future development
Square feet undergoing preconstruction activities (in square feet)	1,900,000
Alexandria Center for Life Science-New York City
Rental properties, construction in progress, and land held for future development
Square feet undergoing preconstruction activities (in square feet)	407,000

The number of the largest projects included in preconstruction. Preconstruction activities include these large properties.

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The amount of rentable square feet related to real estate operating properties.

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The net book value of rental properties.

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- Definition

The amount of square footage related to land held for future development.

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The amount of square footage undergoing development activities in India and China.

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The amount of rentable square feet related to a real estate property undergoing active redevelopment.

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The amount of rentable square feet related to a real estate property undergoing active ground-up development.

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The amount of square footage related to real estate undergoing preconstruction activities (consisting of Building Information Modeling (3-D virtual modeling), design development and construction drawings, sustainability and energy optimization review, budgeting, planning for future site and infrastructure work, and other activities prior to commencement of vertical construction of aboveground shell and core improvements) that are also classified as construction in progress. Our objective with preconstruction is to reduce the time it takes to deliver projects to prospective tenants. Project costs are capitalized as a cost of the project during periods when activities necessary to prepare an asset for its intended use are in progress. If vertical aboveground construction is not initiated at completion of preconstruction activities, the land parcel will be classified as land held for future development. We generally will not commence new development projects for aboveground vertical construction of new laboratory space without first securing significant pre-leasing for such space.

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- Definition

The amount of interest that was capitalized during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 34
-Paragraph 21
-Subparagraph b
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391

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Carrying amount of land available for development.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 7
-Section 03
-Paragraph 1
-Subparagraph d

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.(a),1(d))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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