Organization and Basis of Presentation (Details Textuals) (USD $)	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2012 VIEtransactions sqft VIE NumberofBuildings		Jun. 30, 2012 Segments VIE	Sep. 30, 2012 Segments NumberofTradingDays sqft VIE NumberofBuildings		Dec. 31, 2011 VIE	Sep. 30, 2011
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of buildings	12			12
Number of properties of the company in Washington	10			10
Number of acquisition transactions	4
Rentable square feet	1,532,764			1,532,764
Stabilized Occupancy				95.00%
Percentage of general partnership interest owned by the company in the Operating Partnership	97.60%			97.60%		97.20%	97.10%
Percentage of Common limited partnership interest owned by certain non-affiliated investors and certain directors and officers of the Company in the Operating Partnership	2.40%			2.40%		2.80%	2.90%
Percentage of General partnership interest owned by wholly-owned subsidiary of the Company	1.00%			1.00%
Percentage of limited partnership interest owned by Operating Partnership	99.00%			99.00%
Number of consolidated variable interest entities	4			4		1
Number of days to complete Section 1031 Exchange				180
Variable Interest Entity, Consolidated, Carrying Amount, Assets	$ 317,100,000			$ 317,100,000		$ 108,500,000
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities	94,400,000			94,400,000		7,300,000
Number of Reportable Segments				1
Number of Non-Reportable Segments			1
Real Estate Investment [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Variable Interest Entity, Consolidated, Carrying Amount, Assets	$ 296,656,000			$ 296,656,000		$ 101,352,000
Office Properties Acquisitions [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of buildings	12			12
Development and Redevelopment Project Acquisitions [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of buildings	3			3
Variable Interest Entity, Primary Beneficiary, Aggregated Disclosure [Member] | Office Properties Acquisitions [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of buildings	2			2
Variable Interest Entity, Primary Beneficiary, Aggregated Disclosure [Member] | Development and Redevelopment Project Acquisitions [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of buildings	2			2
Assets Held-for-sale [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of buildings	44	[1]		44	[1]
Rentable square feet	3,721,989	[1]		3,721,989	[1]
Assets Held-For-Sale Industrial Properties [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of buildings	39			39
Assets Held-For-Sale Office Properties [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of buildings	5			5
Lease Up Properties [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of buildings	0			0
Development Properties [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of buildings	1			1
Expected net rentable area	341,000			341,000
Redevelopment Properties [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of buildings	4			4
Rentable square feet	918,000			918,000

[1] Includes all Industrial Properties and five Office Properties (see Note 13 for additional information).