Investments in real estate (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Investments in real estate
Land (related to rental properties)	$ 520,593	$ 510,630
Buildings and building improvements	4,600,499	4,417,093
Other improvements	184,209	185,036
Rental properties	5,305,301	5,112,759
Less: accumulated depreciation	(822,369)	(742,535)
Rental properties, net	4,482,932	4,370,224
Construction in progress ("CIP")/current value-added projects:
Active development in North America	290,289	198,644
Active redevelopment in North America	275,086	281,555
Generic infrastructure/building improvement projects in North America	80,877	92,338
Active development and redevelopment in Asia	97,744	106,775
Construction in progress ("CIP")/current value-added projects	743,996	679,312
Subtotal	5,226,928	5,049,536
Land/future value-added projects:
Land held for future development in North America	324,586	305,981
Land undergoing preconstruction activities (additional CIP) in North America	569,805	574,884
Land held for future development/land undergoing preconstruction activities (additional CIP) in Asia	60,161	35,697
Land/future value-added projects	954,552	916,562
Investment in unconsolidated real estate entity	26,874	42,342
Investments in real estate, net	$ 6,208,354	$ 6,008,440

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Costs of real estate projects undergoing active development.

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Costs of real estate projects undergoing active redevelopment.

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Costs of CIP/current value-added redevelopment and development projects.

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The current amount of expenditures for generic infrastructure or building projects undergoing development.

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Carrying amount as of the balance sheet date of improvements not otherwise specified in the existing taxonomy.

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Represents the aggregate carrying amount of land held for future development and land undergoing preconstruction activities expressed as land/future value-added projects.

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Represents the carrying amount of land available for development and land 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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Represents the carrying amount of land 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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The net book value of rental properties.

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Aggregate of the carrying amounts as of the balance sheet date of investments in building and building improvements.

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Carrying amount as of the balance sheet date of real estate held for productive use. This excludes land held for sale.

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Carrying amount of land available for development.

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The cumulative amount of depreciation for real estate property held for investment purposes.

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The net book value of real estate property held for investment purposes.

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Investments in real estate held solely for investment purposes (income production and capital appreciation).

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The carrying amount of investments in unconsolidated real estate and other joint ventures not separately presented. This includes direct and indirect investments.

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Carrying amount of income producing properties held for rental.

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