Investments in Unconsolidated Joint Ventures investment in Unconsolidated Joint Ventures (Narrative) (Details) (USD $)	9 Months Ended		0 Months Ended			0 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Apr. 10, 2014 501 K Street [Member] sqft	Sep. 30, 2014 501 K Street [Member] payments sqft		Jun. 23, 2014 Annapolis Junction Building Eight [Member] Y	Apr. 30, 2014 Annapolis Junction Building Eight [Member] sqft	Apr. 10, 2014 joint venture partner [Member] 501 K Street [Member]	Apr. 10, 2014 Company [Member] 501 K Street [Member]	Apr. 30, 2014 Company [Member] Annapolis Junction Building Eight [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Description of Variable Rate Basis						LIBOR plus 1.50%
Capital contributions to unconsolidated joint ventures	$ 47,767,000	$ 0	$ 39,000,000				$ 5,400,000
Ownership Percentage				50.00%	[1]			50.00%	50.00%	50.00%
Net Rentable Area (in sf)			520,000				125,000
Construction Loan Facility Borrowing Capacity						$ 26,000,000
Debt Instrument, Basis Spread on Variable Rate						1.50%
Maturity Date						Jun. 23, 2017
Number of extensions						2
Extension Option (in years)						1
Potential additonal payments to joint venture partner				2
Minimum square footage to make a potential additional payment to joint venture partner (in sqft)				520,000

[1] Under the joint venture agreement, the partner will be entitled to up to two additional payments from the venture based on increases in total square footage of the project above 520,000 square feet and achieving certain project returns at stabilization.