The expense recognized in the current period that allocates the cost of a tangible asset used in providing revenue generating services over the asset's useful life.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 28

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.2(d))

-URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688

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- Definition

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

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- Definition

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

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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 45

-Paragraph 4

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=6361293&loc=d3e6812-107765

Reference 2: 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)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

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- Definition

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

+ 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(3))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 7

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Amount of real estate investment property 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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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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- Definition

Carrying amount as of the balance sheet date of improvements having a life longer than one year that were made for the benefit of one or more tenants.

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REAL ESTATE PROPERTIES (Details) (USD $)	3 Months Ended		6 Months Ended
REAL ESTATE PROPERTIES (Details) (USD $)	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Real Estate Properties [Line Items]
Depreciation Expense During the Period	$ 13,494,000	$ 12,899,000	$ 26,551,000	$ 25,977,000
Real Estate Properties
Land	262,063,000		262,063,000		244,199,000
Building and building improvements	1,183,020,000		1,183,020,000		1,102,597,000
Tenant and other improvements	291,384,000		291,384,000		272,981,000
Development	145,531,000		145,531,000		148,255,000
Real estate and development properties	1,881,998,000		1,881,998,000		1,768,032,000
Less accumulated depreciation	(522,520,000)		(522,520,000)		(496,247,000)
Real estate, net	$ 1,359,478,000		$ 1,359,478,000		$ 1,271,785,000
Building [Member]
Real Estate Properties [Line Items]
Property, Plant and Equipment, Useful Life			40 years
Minimum [Member] \| Improvements [Member]
Real Estate Properties [Line Items]
Property, Plant and Equipment, Useful Life			3 years
Maximum [Member] \| Improvements [Member]
Real Estate Properties [Line Items]
Property, Plant and Equipment, Useful Life			15 years