Real Estate Real Estate Values (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Land	$ 4,384,200	$ 4,367,816
Depreciable property Abstract
Investment Building and Building Improvements	14,294,619	14,262,616
Fixtures and Equipment, Gross	1,311,696	1,292,124
Development in Process	185,621	160,190
Land Available for Development	360,955	325,200
Investment in real estate	20,537,091	20,407,946
Real Estate Investment Property, Accumulated Depreciation	(4,658,994)	(4,539,583)
Investment in real estate, net	15,878,097	15,868,363
Projects under development [Member]
Depreciable property Abstract
Land Under Development	76,112	75,646
Development in Process	109,509	84,544
Land held for development [Member]
Depreciable property Abstract
Development in Process	37,938	26,104
Land Available for Development	$ 323,017	$ 299,096

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Carrying amount of land for a real estate project that has not yet been completed.

+ References

No definition available.

+ Details

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- Definition

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

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.10)

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- Definition

Gross amount of fixtures and equipment as of the date of the statement of financial position. Fixtures may include and consist of: machinery, equipment, engines, boilers, incinerators, installed building materials; systems and equipment for the purpose of supplying or distributing heating, cooling, electricity, gas, water, air, or light; antennas, cable, wiring and conduits used in connection with radio, television, security, fire prevention, or fire detection or otherwise used to carry electronic signals; telephone systems and equipment; elevators and related machinery and equipment; fire detection, prevention and extinguishing systems and apparatus; security and access control systems and apparatus; plumbing systems; water heaters, ranges, stoves, microwave ovens, refrigerators, dishwashers, garbage disposers, washers, dryers and other appliances; light fixtures, awnings, storm windows and storm doors; pictures, screens, blinds, shades, curtains and curtain rods; mirrors; cabinets, paneling, rugs and floor and wall coverings; fences, trees and plants; swimming pools; and such other items as defined by the reporting entity.

+ References

No definition available.

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- Definition

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

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.1(d))

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- Definition

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

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.13)

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-Topic 210

-SubTopic 10

-Section 45

-Paragraph 4

-Subparagraph (e)

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- Definition

Carrying amount of land available for development.

+ References

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 7

-Section 03

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-Subparagraph d

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- Definition

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

+ References

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 7

-Section 03

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-Subparagraph (SX 210.7-03.1(3))

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- Definition

Represents a total 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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- Definition

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

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 210

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