Organization and Basis of Presentation (Details Textuals) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013 building VIE	Mar. 31, 2012	Dec. 31, 2012 VIEtransactions VIE
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Stabilized Occupancy	95.00%
Number of properties of the company in Washington	11
Percentage of general partnership interest owned by the company in the Operating Partnership	97.60%	97.50%	97.60%
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%	2.50%	2.40%
Percentage of General partnership interest owned by wholly-owned subsidiary of the Company	1.00%
Percentage of limited partnership interest owned by Operating Partnership	99.00%
Number of consolidated variable interest entities			2
Number of acquisition transactions			2
Variable Interest Entity, Consolidated, Carrying Amount, Assets			$ 337,000,000
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities			111,100,000
Restricted cash	$ 228,079,000	$ (386,000)
Office Properties Acquisitions [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of buildings	2
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			1
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			1