Organization and Basis of Presentation (Details Textuals) (USD $) In Millions, unless otherwise specified	3 Months Ended			3 Months Ended			12 Months Ended
	Mar. 31, 2012 NumberofTradingDays RSF NumberofBuildings VIE	Dec. 31, 2011 VIE	Mar. 31, 2011	Mar. 31, 2012 Office Properties [Member] RSF numberofaquisitiontransactions NumberofBuildings	Mar. 31, 2012 Redevelopment Properties [Member] RSF NumberofBuildings	Mar. 31, 2012 Lease Up Properties [Member] NumberofBuildings	Dec. 31, 2011 Assets Held-for-sale [Member] RSF NumberofBuildings
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of properties of the company in Washington	6
Number of Buildings	7			7	4	0	2
Number of acquisition transactions				1
Rentable square feet	374,139			374,139	918,000		254,000
Stabilized Occupancy	95.00%
Percentage of general partnership interest owned by the company in the Operating Partnership	97.50%	97.20%	96.80%
Percentage of Common limited partnership interest owned by certain non-affiliated investors and certain directors and officers of the Company in the Operating Partnership	2.50%	2.80%	3.20%
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	1	1
Number of days to complete Section 1031 Exchange	180
Variable Interest Entity, Consolidated, Carrying Amount, Assets	$ 172.2	$ 108.5
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities	$ 10.4	$ 7.3