Discontinued Operations Income from Office Property Disposition and Held for Sale (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended	6 Months Ended		12 Months Ended		6 Months Ended		12 Months Ended		6 Months Ended		12 Months Ended		6 Months Ended		12 Months Ended		6 Months Ended		12 Months Ended		6 Months Ended		12 Months Ended		6 Months Ended		12 Months Ended		6 Months Ended		12 Months Ended				6 Months Ended				6 Months Ended								6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Falls Pointe [Member]		Dec. 31, 2012 Falls Pointe [Member] sqft		Jun. 30, 2013 111 East Wacker [Member]		Dec. 31, 2012 111 East Wacker [Member] sqft		Jun. 30, 2013 Renaissance Center [Member]		Dec. 31, 2012 Renaissance Center [Member] sqft		Jun. 30, 2013 Overlook II [Member]		Dec. 31, 2012 Overlook II [Member] sqft		Jun. 30, 2013 Wink Building [Member]		Dec. 31, 2012 Wink Building [Member] sqft		Jun. 30, 2013 Ashford/Peachtree [Member]		Dec. 31, 2012 Ashford/Peachtree [Member] sqft		Jun. 30, 2013 Non-Core Assets [Member]		Dec. 31, 2012 Non-Core Assets [Member] sqft		Jun. 30, 2013 Sugar Grove [Member]		Dec. 31, 2012 Sugar Grove [Member] sqft		Dec. 31, 2012 2012 Dispositions [Member] sqft		Jun. 30, 2013 Atrium at Stoneridge [Member]		Mar. 20, 2013 Atrium at Stoneridge [Member] sqft		Jun. 30, 2013 2013 Dispositions [Member] sqft		Jun. 30, 2013 Waterstone [Member] sqft		Jun. 30, 2013 Meridian [Member] sqft		Jun. 30, 2013 Waterstone and Meridian [Member] sqft		Jun. 30, 2013 Subsequent Event [Member] Waterstone [Member]		Jun. 30, 2013 Subsequent Event [Member] Meridian [Member]		Jul. 10, 2013 Subsequent Event [Member] Waterstone and Meridian [Member]
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	[3]	Atlanta, GA	[3]
Area of real estate property (in square feet)								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	[3]	97,000	[3]	190,000	[3]
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	[3]	Jul. 10, 2013	[3]
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]			$ 2,966,000	[2]
Net Book Value of Real Estate	1,729,092,000		1,729,092,000		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]			2,424,000	[2]
Gross sales price of assets under agreement to sell																																														3,400,000	[3]	6,800,000	[3]	10,200,000	[3]					10,200,000
Gain on sale of real estate from discontinued operations	0	3,197,000	542,000	8,772,000				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]			542,000	[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 our proportionate share.

[2] Total gain on the sale of real estate in discontinued operations recognized during the six months ended June 30, 2013 was $542,000.

[3] Gains and losses on assets held for sale are expected to be finalized upon sale and reflected in the nine months ended September 30, 2013 financial statements.