Property Transactions (Properties Held for Sale) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended						12 Months Ended
	Dec. 31, 2012 The Avenue Forsyth Suburban Atlanta, GA [Member] sqft	Dec. 31, 2012 The Avenue Collierville [Member] sqft	Dec. 31, 2012 The Avenue Webb Gin Suburban Atlanta, GA [Member] sqft	Dec. 31, 2012 Galleria 75 [Member] sqft	Dec. 31, 2012 Cosmopolitan Center [Member] sqft	Dec. 31, 2012 Inhibitex Suburban Atlanta, GA [Member] sqft	Dec. 31, 2011 King Mill Distribution Park- Building 3 Suburban Atlanta, GA [Member] sqft	Dec. 31, 2011 Lakeside Ranch Business Park - Building 20 Dallas, TX [Member] sqft	Dec. 31, 2011 Jefferson Mill Business Park - Building A Suburban Atlanta, GA [Member] sqft	Dec. 31, 2011 One Georgia Center Atlanta, GA [Member] sqft	Dec. 31, 2010 San Jose MarketCenter, San Jose CA [Member] sqft	Dec. 31, 2010 8995 Westside Parkway, Atlanta GA [Member] sqft
Summary of sales in discontinued operations
Area of disposed real estate assets (square feet)	524,000	511,000	322,000	111,000	51,000	51,000	796,000	749,000	459,000	376,000	213,000	51,000
Sales price of property held for sale, discontinued operations	$ 119,000	$ 55,000	$ 59,600	$ 9,200	$ 7,000		$ 28,300	$ 28,400	$ 22,000	$ 48,600	$ 85,000	$ 3,200