Discontinued Operations Income from Office Property Disposition and Held for Sale (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended	9 Months Ended		12 Months Ended		9 Months Ended		12 Months Ended		9 Months Ended		12 Months Ended		9 Months Ended		12 Months Ended		9 Months Ended		12 Months Ended		9 Months Ended		12 Months Ended		9 Months Ended		12 Months Ended		9 Months Ended		12 Months Ended				9 Months Ended				9 Months Ended		0 Months Ended	9 Months Ended
	Sep. 30, 2013 sqft	Sep. 30, 2012	Sep. 30, 2013 sqft	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Falls Pointe		Dec. 31, 2012 Falls Pointe sqft		Sep. 30, 2013 111 East Wacker		Dec. 31, 2012 111 East Wacker sqft		Sep. 30, 2013 Renaissance Center		Dec. 31, 2012 Renaissance Center sqft		Sep. 30, 2013 Overlook II		Dec. 31, 2012 Overlook II sqft		Sep. 30, 2013 Wink Building		Dec. 31, 2012 Wink Building sqft		Sep. 30, 2013 Ashford/Peachtree		Dec. 31, 2012 Ashford/Peachtree sqft		Sep. 30, 2013 Non-Core Assets		Dec. 31, 2012 Non-Core Assets sqft		Sep. 30, 2013 Sugar Grove		Dec. 31, 2012 Sugar Grove sqft		Dec. 31, 2012 2012 Dispositions sqft		Sep. 30, 2013 Atrium at Stoneridge sqft		Mar. 20, 2013 Atrium at Stoneridge sqft		Sep. 30, 2013 Waterstone sqft	Sep. 30, 2013 Meridian sqft	Jul. 17, 2013 Bank of America Plaza	Sep. 30, 2013 Bank of America Plaza sqft	Sep. 30, 2013 2013 Dispositions sqft
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Location						Atlanta, GA	[1]			Chicago, IL	[1]			Memphis, TN	[1]			Atlanta, GA	[1]			New Orleans, LA	[1]			Atlanta, GA	[1]			Various	[1]			Houston, TX	[1]					Columbia, SC	[2]			Atlanta, GA	Atlanta, GA		Nashville, TN
Area of real estate property (in square feet)	640,000		640,000					107,000	[1]			1,013,000	[1]			189,000	[1]			260,000	[1]			32,000	[1]			321,000	[1]			1,932,000	[1]			124,000	[1]	3,978,000	[1]	108,000	[2]	108,000	[2]	93,000	97,000		436,000	734,000	[2]
Date of sale						Jan. 06, 2012	[1]			Jan. 09, 2012	[1]			Mar. 01, 2012	[1]			Apr. 30, 2012	[1]			Jun. 08, 2012	[1]			Jul. 01, 2012	[1]							Oct. 23, 2012	[1]					Mar. 20, 2013	[2]			Jul. 10, 2013	Jul. 10, 2013		Jul. 17, 2013
Net Sales Price								$ 5,824,000	[1]			$ 153,240,000	[1]			$ 27,661,000	[1]			$ 29,467,000	[1]			$ 705,000	[1]			$ 29,440,000	[1]			$ 125,486,000	[1]			$ 10,303,000	[1]	$ 382,126,000	[1]	$ 2,966,000	[2]			$ 3,247,000	$ 6,615,000		$ 41,093,000	$ 53,921,000	[2]
Net Book Value of Real Estate	1,652,930,000		1,652,930,000		1,562,717,000			4,467,000	[1]			153,237,000	[1]			24,629,000	[1]			28,689,000	[1]			803,000	[1]			28,148,000	[1]			122,157,000	[1]			7,057,000	[1]	369,187,000	[1]	2,424,000	[2]			3,207,000	6,560,000		29,643,000	41,834,000	[2]
Gain on sale of real estate from discontinued operations	11,545,000	995,000	12,087,000	9,767,000				1,357,000	[1]			3,000	[1]			3,032,000	[1]			778,000	[1]			(98,000)	[1]			1,292,000	[1]			3,329,000	[1]			3,246,000	[1]	12,939,000	[1]	542,000	[2]			40,000	55,000	11,500,000	11,450,000	12,087,000	[2]
Proportionate share of gain on sale of assets owned by joint venture					$ 8,100,000

[1] Total gain on the sale of real estate in discontinued operations recognized for the year ended December 31, 2012 was $12.9 million, of which $8.1 million was the Company's proportionate share.

[2] Total gain on the sale of real estate in discontinued operations recognized during the nine months ended September 30, 2013 was $12.1 million.