Property Transactions Property Acquisition (Details) (USD $) In Thousands, unless otherwise specified			12 Months Ended
	Mar. 31, 2013 Inhibitex Suburban Atlanta [Member] sqft	Dec. 31, 2012 Inhibitex Suburban Atlanta [Member] sqft	Dec. 31, 2012 Avenue Forsyth Suburban Atlanta [Member] sqft	Dec. 31, 2012 The Avenue Collierville [Member] sqft	Dec. 31, 2012 Avenue Webb Gin Suburban Atlanta [Member] sqft	Dec. 31, 2012 Galleria 75 [Member] sqft	Dec. 31, 2012 Cosmopolitan Center [Member] sqft
Business Acquisition [Line Items]
Area of Real Estate Property	51,000	51,000	524,000	511,000	322,000	111,000	51,000
Sales Price of Property Sold Discontinued Operations			$ 119,000	$ 55,000	$ 59,600	$ 9,200	$ 7,000