Subsequent Events (Details) (USD $)	3 Months Ended			0 Months Ended				1 Months Ended	3 Months Ended	0 Months Ended	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011	May 03, 2012 12310 Sunrise Valley Drive [Member]	Apr. 30, 2012 510 Madison Avenue [Member]	May 01, 2012 Bedford Business Park [Member]	May 08, 2012 Atm Program [Member]	May 08, 2012 Atm Program [Member]	Mar. 31, 2012 Atm Program [Member]	Apr. 02, 2012 One Freedom Square [Member]	Mar. 31, 2012 One Freedom Square [Member]	May 01, 2012 Office/Technical Properties [ Member] Bedford Business Park [Member]	May 01, 2012 Class A Office Properties [ Member] Bedford Business Park [Member]
Subsequent Event [Line Items]
Shares issued	149,463,241		148,186,511				643,600	643,600	1,048,800
At the market stock offering program, gross proceeds								$ 68,400,000	$ 110,500,000
At the market stock offering program, net proceeds	109,330,000	345,369,000						67,700,000	109,300,000
At the market stock offering program remaining amount available for issuance							376,200,000		444,600,000
Repayments of mortgage notes payable	150,519,000	8,777,000								65,100,000
Interest at a fixed rate										7.75%
Maturity date											Jun. 30, 2012
Penalty for early repayment of debt										0
Percentage of office component of the development project placed in-service				100.00%	100.00%
Disposition price						$ 62,800,000
Number of real estate properties												2	1
Square footage of real estate property				268,000	347,000	470,000