Investments in real estate (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Investments in real estate
Land (related to rental properties)	$ 506,823	$ 510,630
Buildings and building improvements	4,682,998	4,417,093
Other improvements	184,301	185,036
Rental properties	5,374,122	5,112,759
Less: accumulated depreciation	(854,332)	(742,535)
Rental properties, net	4,519,790	4,370,224
Construction in progress ("CIP")/current value-added projects:
Subtotal	6,300,027	6,008,440
Land/future value-added projects:
Investment in unconsolidated real estate entity	26,998	42,342
Investments in real estate, net	6,300,027	6,008,440
Construction in progress ("CIP")/current value-added projects:
Construction in progress ("CIP")/current value-added projects:
Development in process	750,165	679,312
Active development \| North America
Construction in progress ("CIP")/current value-added projects:
Development in process	304,619	198,644
Active redevelopment \| North America
Construction in progress ("CIP")/current value-added projects:
Development in process	277,506	281,555
Generic infrastructure/building improvement projects \| North America
Construction in progress ("CIP")/current value-added projects:
Development in process	72,739	92,338
Active development and redevelopment \| Asia
Construction in progress ("CIP")/current value-added projects:
Development in process	95,301	106,775
Land/future value-added projects:
Land/future value-added projects:
Land available for development	1,003,074	916,562
Land held for future development \| North America
Land/future value-added projects:
Land available for development	326,932	305,981
Land undergoing preconstruction activities (additional CIP) \| North America
Land/future value-added projects:
Land available for development	597,631	574,884
Land held for future development/land undergoing preconstruction activities (additional CIP) \| Asia
Land/future value-added projects:
Land available for development	78,511	35,697
Rental properties, net and Construction in progress ("CIP")/current value-added projects
Construction in progress ("CIP")/current value-added projects:
Subtotal	5,269,955	5,049,536
Land/future value-added projects:
Investments in real estate, net	$ 5,269,955	$ 5,049,536

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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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The net book value of rental properties.

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The current amount of expenditures for a real estate project that has not yet been completed.

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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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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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Amount of real estate investment property, net of accumulated depreciation, which may include the following: (1) land available-for-sale; (2) land available-for-development; (3) investments in building and building improvements; (4) tenant allowances; (5) developments in-process; (6) rental properties; and (7) other real estate investments.

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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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