Debt (Details) (USD $)	3 Months Ended		9 Months Ended		0 Months Ended		9 Months Ended	0 Months Ended				0 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Mar. 24, 2011 Tower Shops January 19 2011 [Member]	Jan. 19, 2011 Tower Shops January 19 2011 [Member]	Sep. 30, 2011 Tower Shops January 19 2011 [Member]	Apr. 29, 2011 Federal Plaza [Member]	Jun. 01, 2011 Tyson Station [Member]	Feb. 15, 2011 4.5%SeniorNotes [Member]	Sep. 30, 2011 Existing Revolving Credit Facility [Member]	Jul. 07, 2011 New Revolving Credit Facility [Member]
Mortgage loan face amount						$ 41,000,000
Mortgage loan fair value						42,900,000
Mortgage loan interest rate						6.52%				4.50%
Debt Instrument, Maturity Date						Jul. 01, 2015		Jun. 01, 2011	Sep. 01, 2011
Percentage of prepayment premium on mortgage loan						3.00%
Repayment of mortgage loan					41,000,000			31,700,000	5,600,000
Prepayment premium on mortgage loan					1,200,000
Income from early extinguishment of debt			296,000	(2,801,000)
Unamortized debt premium							1,700,000
Unsecured revolving credit facility											300,000,000	400,000,000
Repayment of senior notes			75,000,000							75,000,000
Revolving credit facility, maximum outstanding	219,000,000		265,000,000
Line of credit facility, weighted average amount outstanding	184,600,000		186,000,000
Line of credit facility weighted average interest rate before amortization of debt fees	1.31%		0.89%
Line of credit facility, amount outstanding	$ 158,000,000		$ 158,000,000
Extension Option Period												1 year
Debt Instrument, Interest Rate Terms												LIBOR plus 115 basis points
Basis Points Spread Over LIBOR												115
Line of Credit Facility, Expiration Date												July 6, 2015