Discontinued Operations - Schedule of Property Dispositions (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended						0 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Mar. 20, 2013	Jul. 17, 2013
Significant Acquisitions and Disposals [Line Items]
Net Book Value of Real Estate	$ 3,024,271		$ 2,316,795
Gain on Sale	10,463		32,493		12,938
Non-cash impairment loss on real estate	11,700		0		5,700
Falls Pointe
Significant Acquisitions and Disposals [Line Items]
Location					Atlanta, GA
Date of Sale					Jan. 06, 2012
Net Sales Price					5,824
Net Book Value of Real Estate					4,467
Gain on Sale					1,357
Non-cash impairment loss on real estate					0
111 East Wacker
Significant Acquisitions and Disposals [Line Items]
Location					Chicago, IL
Date of Sale					Jan. 09, 2012
Net Sales Price					153,240
Net Book Value of Real Estate					153,237
Gain on Sale					3
Non-cash impairment loss on real estate					19,050
Renaissance Center
Significant Acquisitions and Disposals [Line Items]
Location					Memphis, TN
Date of Sale					Mar. 01, 2012
Net Sales Price					27,661
Net Book Value of Real Estate					24,629
Gain on Sale					3,032
Non-cash impairment loss on real estate					9,200
Non-Core Assets
Significant Acquisitions and Disposals [Line Items]
Location					Various
Net Sales Price					125,486
Net Book Value of Real Estate					122,157
Gain on Sale					3,329
Non-cash impairment loss on real estate					51,889
Overlook II
Significant Acquisitions and Disposals [Line Items]
Location					Atlanta, GA
Date of Sale					Apr. 30, 2012
Net Sales Price					29,467
Net Book Value of Real Estate					28,689
Gain on Sale					778
Non-cash impairment loss on real estate					10,500
Wink
Significant Acquisitions and Disposals [Line Items]
Location					New Orleans, LA
Date of Sale					Jun. 08, 2012
Net Sales Price					705
Net Book Value of Real Estate					803
Gain on Sale					(98)
Non-cash impairment loss on real estate					0
Ashford Center/Peachtree Ridge
Significant Acquisitions and Disposals [Line Items]
Location					Atlanta, GA
Date of Sale					Jul. 01, 2012
Net Sales Price					29,440
Net Book Value of Real Estate					28,148
Gain on Sale					1,292
Non-cash impairment loss on real estate					17,200
Sugar Grove
Significant Acquisitions and Disposals [Line Items]
Location					Houston, TX
Date of Sale					Oct. 23, 2012
Net Sales Price					10,303
Net Book Value of Real Estate					7,058
Gain on Sale					3,245
Non-cash impairment loss on real estate					0
2012 Dispositions
Significant Acquisitions and Disposals [Line Items]
Net Sales Price					382,126	[1]
Net Book Value of Real Estate					369,188	[1]
Gain on Sale					12,938	[1]
Non-cash impairment loss on real estate					107,839	[1]
Waterstone and Meridian
Significant Acquisitions and Disposals [Line Items]
Gain on Sale			5,600
Non-cash impairment loss on real estate			4,600
Atrium at Stoneridge
Significant Acquisitions and Disposals [Line Items]
Location			Columbia, SC	[2]
Date of Sale			Mar. 20, 2013	[2]
Net Sales Price			2,966	[2]
Net Book Value of Real Estate			2,424	[2]
Gain on Sale			542	[2]			542
Non-cash impairment loss on real estate			3,500	[2]
Waterstone (3)
Significant Acquisitions and Disposals [Line Items]
Location			Atlanta, GA	[3]
Date of Sale			Jul. 10, 2013	[3]
Net Sales Price			3,247	[3]
Net Book Value of Real Estate			3,207	[3]
Gain on Sale			40	[3]
Non-cash impairment loss on real estate			3,000	[3]
Meridian (3)
Significant Acquisitions and Disposals [Line Items]
Location			Atlanta, GA	[3]
Date of Sale			Jul. 10, 2013	[3]
Net Sales Price			6,615	[3]
Net Book Value of Real Estate			6,560	[3]
Gain on Sale			55	[3]
Non-cash impairment loss on real estate			1,600	[3]
Bank of America Plaza
Significant Acquisitions and Disposals [Line Items]
Location			Nashville, TN
Date of Sale			Jul. 17, 2013
Net Sales Price			41,093
Net Book Value of Real Estate			29,643
Gain on Sale			11,450					11,500
Non-cash impairment loss on real estate			0
Lakewood II
Significant Acquisitions and Disposals [Line Items]
Location			Atlanta, GA
Date of Sale			Oct. 31, 2013
Net Sales Price			10,240
Net Book Value of Real Estate			4,403
Gain on Sale			5,837
Non-cash impairment loss on real estate			0
Carmel Crossing
Significant Acquisitions and Disposals [Line Items]
Location			Charlotte, NC
Date of Sale			Nov. 08, 2013
Net Sales Price			36,673
Net Book Value of Real Estate			22,104
Gain on Sale			14,569
Non-cash impairment loss on real estate			0
2013 Dispositions
Significant Acquisitions and Disposals [Line Items]
Net Sales Price			100,834	[4]
Net Book Value of Real Estate			68,341	[4]
Gain on Sale			32,493	[4]
Non-cash impairment loss on real estate			8,100	[4]
Woodbranch Building
Significant Acquisitions and Disposals [Line Items]
Location	Houston, TX
Date of Sale	Jan. 14, 2014
Net Sales Price	14,424
Net Book Value of Real Estate	4,450
Gain on Sale	9,974
Non-cash impairment loss on real estate	0
Mesa Corporate Center
Significant Acquisitions and Disposals [Line Items]
Location	Phoenix, AZ	[3]
Date of Sale	Jan. 31, 2014	[3]
Net Sales Price	12,257	[3]
Net Book Value of Real Estate	11,768	[3]
Gain on Sale	489	[3]
Non-cash impairment loss on real estate	5,600	[3]
2014 Dispositions
Significant Acquisitions and Disposals [Line Items]
Net Sales Price	26,681	[5]
Net Book Value of Real Estate	16,218	[5]
Gain on Sale	10,463	[5]
Non-cash impairment loss on real estate	5,600	[5]
Parent | 2012 Dispositions
Significant Acquisitions and Disposals [Line Items]
Gain on Sale					8,100
Parent | 2013 Dispositions
Significant Acquisitions and Disposals [Line Items]
Gain on Sale			$ 18,200

[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] The impairment loss on real estate in connection with Atrium at Stoneridge was recognized in discontinued operations during the year ended December 31, 2012.

[3] The impairment loss on real estate recognized in discontinued operations during the year ended December 31, 2013 was comprised of a $4.6 million loss in connection with the Company's Waterstone and Meridian properties that were sold in 2013 and a $5.6 million loss in connection with the valuation of Mesa Corporate Center based on its estimated fair value of the asset, and which was classified as held for sale at December 31, 2013.

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

[5] With the adoption of ASU 2014-08, "Reporting Discontinued Operations and Disposals of Components of an Entity" effective January 1, 2014, the Company's 2014 sales of the Woodbranch Building and Mesa Corporate Center are included in discontinued operations for the year ended December 31, 2014 as these properties were previously classified as held for sale at December 31, 2013. The Company's 2014 sales of Schlumberger Building and 525 North Tryon are included in the continuing operations section of the consolidated statement of operations and comprehensive income (loss) as they were not previously classified as held for sale in any prior period financial statements.