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

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)

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

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- Definition

The charge against earnings in the period to reduce the carrying amount of real property to fair value.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 144

-Paragraph 26

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

-SubTopic 10

-Section 45

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-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 360

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6391110&loc=d3e2921-110230

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- Definition

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

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-Paragraph 1

-Subparagraph (SX 210.7-03.(a),1(d))

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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.

+ 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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Real Estate Investments Continuing Operations (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Real Estate Properties [Line Items]
Real estate investment property, at cost	$ 2,406,783,000	$ 2,051,936,000
Held for development	43,089,000	26,240,000
Real estate impairment	14,526,000	0	0
Land Available for Development	9,400,000
Office [Member]
Real Estate Properties [Line Items]
Real estate investment property, at cost	1,268,136,000	980,263,000
Medical Office [Member]
Real Estate Properties [Line Items]
Real estate investment property, at cost	404,793,000	398,559,000
Retail [Member]
Real Estate Properties [Line Items]
Real estate investment property, at cost	408,897,000	351,395,000
Multifamily [Member]
Real Estate Properties [Line Items]
Real estate investment property, at cost	324,957,000	321,719,000
Office [Member]
Real Estate Properties [Line Items]
Held for development	8,953,000	20,172,000
Medical Office [Member]
Real Estate Properties [Line Items]
Held for development	5,758,000	5,463,000
Retail [Member]
Real Estate Properties [Line Items]
Held for development	576,000	546,000
Multifamily [Member]
Real Estate Properties [Line Items]
Held for development	27,802,000	59,000
Dulles Station II [Member]
Real Estate Properties [Line Items]
Real estate impairment	14,526,000
Land Available for Development	$ 12,100,000