The calculated weighted-average amortization period of a major class of finite-lived intangible assets acquired during the current period either individually or as part of a group of assets (in either an asset acquisition or business combination). A major class is composed of intangible assets that can be grouped together because they are similar, either by their nature or by their use in the operations of a company.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 142

-Paragraph 44

-Subparagraph a(3)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 350

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (a)(3)

-URI http://asc.fasb.org/extlink&oid=6905858&loc=d3e16265-109275

Real Estate Acquisitions And Discontinued Operations (Narrative) (Details) (USD $)	0 Months Ended	3 Months Ended	9 Months Ended			9 Months Ended					0 Months Ended	9 Months Ended
	Sep. 02, 2011	Sep. 30, 2011	Sep. 30, 2011	Sep. 30, 2010	Aug. 05, 2011	Sep. 30, 2011 Tenant Origination Costs [Member] months	Sep. 30, 2011 Leasing Commissions/Absorption Costs [Member] months	Sep. 30, 2011 Net Lease Intangible Assets [Member] months	Sep. 30, 2011 Net Lease Intangible Liabilities [Member] months	Sep. 30, 2011 Dulles Station, Phase I [Member]	Jun. 15, 2011 650 North Glebe Road [Member]	Sep. 30, 2011 John Marshall II and Olney Village Center [Member]
Real Estate Properties [Line Items]
Weighted remaining average life, in months						65	54	68	65
Payments to Acquire Businesses, Net of Cash Acquired											$ 11,800,000
Contract purchase price		371,600,000	371,600,000
Acquisition cost			281,701,000	68,000,000
Notes Assumed			(78,500,000)									76,700,000
Credits received at settlement			1,300,000
Number of purchase and sale agreements entered		5	5		5
Number of office assets					2
Number of purchase and sale agreements closed on September 2, 2011	3
Number of remaining purchase and sale agreements closed at end of period		2	2
Aggregate purchase price			350,000,000
Proceeds from sale of real estate	235,800,000
Gain on sale of real estate		56,600,000
Impairment charge										$ 600,000