Discontinued Operations Income from Office Property Disposition and Held for Sale (Details) (USD $)	3 Months Ended			3 Months Ended										0 Months Ended	3 Months Ended
	Mar. 31, 2014 sqft	Mar. 31, 2013	Dec. 31, 2013	Mar. 31, 2013 Atrium at Stoneridge sqft	Mar. 31, 2013 Waterstone sqft	Mar. 31, 2013 Meridian sqft	Mar. 31, 2013 Bank of America Plaza sqft	Mar. 31, 2013 Lakewood II sqft	Mar. 31, 2013 Carmel Crossing sqft	Mar. 31, 2013 2013 Dispositions sqft		Mar. 31, 2014 Woodbranch sqft	Jan. 17, 2014 Woodbranch sqft	Jan. 31, 2014 Mesa Corporate Center sqft	Mar. 31, 2014 Mesa Corporate Center sqft	Mar. 31, 2014 2014 Dispositions sqft
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Location				Columbia, SC	Atlanta, GA	Atlanta, GA	Nashville, TN	Atlanta, GA	Charlotte, NC			Houston, TX			Phoenix, AZ
Area of real estate property (in square feet)	3,073,000			108,000	93,000	97,000	436,000	123,000	326,000	1,183,000		109,000	109,000	106,000	106,000	215,000
Date of sale				Mar. 20, 2013	Jul. 10, 2013	Jul. 10, 2013	Jul. 17, 2013	Oct. 31, 2013	Nov. 08, 2013			Jan. 17, 2014			Jan. 31, 2014
Net Sales Price				$ 2,966,000	$ 3,247,000	$ 6,615,000	$ 41,093,000	$ 10,240,000	$ 36,673,000	$ 100,834,000		$ 14,424,000			$ 12,257,000	$ 26,681,000
Real estate, net	2,329,405,000		2,316,795,000	2,424,000	3,207,000	6,560,000	29,643,000	4,403,000	22,104,000	68,341,000		4,450,000			11,768,000	16,218,000
Gain on sale of real estate from discontinued operations	10,463,000	542,000		542,000	40,000	55,000	11,450,000	5,837,000	14,569,000	32,493,000	[1]	9,974,000		489,000	489,000	10,463,000
Proportionate share of gain on sale of assets owned by joint venture										$ 18,200,000

[1] Total gain on the sale of real estate in discontinued operations recognized for the year ended December 31, 2013 was $32.5 million, of which $18.2 million was the Company's proportionate share.