Subsequent Events (Details) (USD $)	3 Months Ended		0 Months Ended	1 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Apr. 04, 2012 ATM Equity Offering Sales [Member]	Apr. 25, 2012 Tampa, FL Office Property Disposition [Member]	Apr. 27, 2012 Cary, NC Office Property Acquisition [Member] sqft
Subsequent Event [Line Items]
Number of Common Stock sold at the market during period (in shares)	785,500		307,900
Average price of Common Stock sold at the market during period (in dollars per share)	$ 32.82		$ 32.93
Net proceeds of Common Stock sold at the market during period	$ 25,400,000		$ 10,000,000
Gross proceeds from the sale of real estate				9,500,000
Gains on disposition of discontinued operations	5,134,000	0		1,400,000
Rentable square feet of acquisition (in sq ft)					178,300
Acquisition purchase price					26,000,000
Joint venture's net proceeds received from disposition of property					$ 25,300,000