Real Estate and Accumulated Depreciation (Schedule of Properties) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Real Estate and Accumulated Depreciation
Gross amount at which carried at close of period, total	$ 2,619,488	$ 2,164,815	$ 997,323	$ 1,253,414
Accumulated depreciation	324,543	257,151	258,258	289,473
Operating properties
Real Estate and Accumulated Depreciation
Encumbrances	652,144
Initial cost to company, land and improvements	458,293
Initial cost to company, building and improvements	1,701,743
Costs capitalized subsequent to acquisition, land and improvements	7,376
Costs capitalized subsequent to acquisition, building and improvements	338,815
Gross amount at which carried at close of period, land and improvements	465,669
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	2,040,558
Gross amount at which carried at close of period, total	2,506,227
Accumulated depreciation	324,543
Impairment moved to discontinued operations
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	66,623
Initial cost to company, building and improvements	0
Costs capitalized subsequent to acquisition, land and improvements	(44,977)
Costs capitalized subsequent to acquisition, building and improvements	0
Gross amount at which carried at close of period, land and improvements	21,646
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	0
Gross amount at which carried at close of period, total	21,646
Accumulated depreciation	0
Commercial Real Estate [Member] | Impairment moved to discontinued operations
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	47,382
Initial cost to company, building and improvements	0
Costs capitalized subsequent to acquisition, land and improvements	(35,594)
Costs capitalized subsequent to acquisition, building and improvements	0
Gross amount at which carried at close of period, land and improvements	11,788
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	0
Gross amount at which carried at close of period, total	11,788
Accumulated depreciation	0
Residential Lots [Member] | Impairment moved to discontinued operations
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	19,241
Initial cost to company, building and improvements	0
Costs capitalized subsequent to acquisition, land and improvements	(9,383)
Costs capitalized subsequent to acquisition, building and improvements	0
Gross amount at which carried at close of period, land and improvements	9,858
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	0
Gross amount at which carried at close of period, total	9,858
Accumulated depreciation	0
Life on which depreciation in statement of operations is computed (b)	0 years
Greenway Plaza Houston, TX [Member] | Operating Properties Office [Member]
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	273,651
Initial cost to company, building and improvements	595,547
Costs capitalized subsequent to acquisition, land and improvements	0
Costs capitalized subsequent to acquisition, building and improvements	34,018
Gross amount at which carried at close of period, land and improvements	273,651
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	629,565
Gross amount at which carried at close of period, total	903,216
Accumulated depreciation	50,698
Life on which depreciation in statement of operations is computed (b)	30 years
One Ninety One Peachtree Tower Atlanta [Member] | Operating Properties Office [Member]
Real Estate and Accumulated Depreciation
Encumbrances	100,000
Initial cost to company, land and improvements	5,355
Initial cost to company, building and improvements	141,012
Costs capitalized subsequent to acquisition, land and improvements	4,034
Costs capitalized subsequent to acquisition, building and improvements	87,100
Gross amount at which carried at close of period, land and improvements	9,389
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	228,112
Gross amount at which carried at close of period, total	237,501
Accumulated depreciation	77,637
Life on which depreciation in statement of operations is computed (b)	40 years
Terminus 100 Atlanta, GA [Member] | Operating Properties Office [Member]
Real Estate and Accumulated Depreciation
Encumbrances	185,109
Initial cost to company, land and improvements	87,264
Initial cost to company, building and improvements	129,347
Costs capitalized subsequent to acquisition, land and improvements	0
Costs capitalized subsequent to acquisition, building and improvements	24,033
Gross amount at which carried at close of period, land and improvements	87,264
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	153,380
Gross amount at which carried at close of period, total	240,644
Accumulated depreciation	15,561
Life on which depreciation in statement of operations is computed (b)	42 years
Promenade Atlanta, GA [Member] | Operating Properties Office [Member]
Real Estate and Accumulated Depreciation
Encumbrances	110,946
Initial cost to company, land and improvements	13,439
Initial cost to company, building and improvements	102,790
Costs capitalized subsequent to acquisition, land and improvements	0
Costs capitalized subsequent to acquisition, building and improvements	31,003
Gross amount at which carried at close of period, land and improvements	13,439
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	133,793
Gross amount at which carried at close of period, total	147,232
Accumulated depreciation	18,735
Life on which depreciation in statement of operations is computed (b)	34 years
The American Cancer Society Center Atlanta, GA [Member] | Operating Properties Office [Member]
Real Estate and Accumulated Depreciation
Encumbrances	131,083
Initial cost to company, land and improvements	5,226
Initial cost to company, building and improvements	67,370
Costs capitalized subsequent to acquisition, land and improvements	0
Costs capitalized subsequent to acquisition, building and improvements	31,319
Gross amount at which carried at close of period, land and improvements	5,226
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	98,689
Gross amount at which carried at close of period, total	103,915
Accumulated depreciation	63,724
Life on which depreciation in statement of operations is computed (b)	25 years
816 Congress Austin, TX [Member] | Operating Properties Office [Member]
Real Estate and Accumulated Depreciation
Encumbrances	85,000
Initial cost to company, land and improvements	6,817
Initial cost to company, building and improvements	89,891
Costs capitalized subsequent to acquisition, land and improvements	3,282
Costs capitalized subsequent to acquisition, building and improvements	7,903
Gross amount at which carried at close of period, land and improvements	10,099
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	97,794
Gross amount at which carried at close of period, total	107,893
Accumulated depreciation	6,203
Life on which depreciation in statement of operations is computed (b)	42 years
2100 Ross Avenue [Member] | Operating Properties Office [Member]
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	5,987
Initial cost to company, building and improvements	45,739
Costs capitalized subsequent to acquisition, land and improvements	0
Costs capitalized subsequent to acquisition, building and improvements	33,854
Gross amount at which carried at close of period, land and improvements	5,987
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	79,593
Gross amount at which carried at close of period, total	85,580
Accumulated depreciation	10,427
Life on which depreciation in statement of operations is computed (b)	35 years
The Points at Waterview Suburban Dallas, TX [Member] | Operating Properties Office [Member]
Real Estate and Accumulated Depreciation
Encumbrances	14,598
Initial cost to company, land and improvements	2,558
Initial cost to company, building and improvements	22,910
Costs capitalized subsequent to acquisition, land and improvements	86
Costs capitalized subsequent to acquisition, building and improvements	10,512
Gross amount at which carried at close of period, land and improvements	2,644
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	33,422
Gross amount at which carried at close of period, total	36,066
Accumulated depreciation	19,736
Life on which depreciation in statement of operations is computed (b)	25 years
Meridian Mark Plaza Atlanta, GA [Member] | Operating Properties Office [Member]
Real Estate and Accumulated Depreciation
Encumbrances	25,408
Initial cost to company, land and improvements	2,219
Initial cost to company, building and improvements	0
Costs capitalized subsequent to acquisition, land and improvements	0
Costs capitalized subsequent to acquisition, building and improvements	27,105
Gross amount at which carried at close of period, land and improvements	2,219
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	27,105
Gross amount at which carried at close of period, total	29,324
Accumulated depreciation	16,688
Life on which depreciation in statement of operations is computed (b)	30 years
555 North Point Center East Suburban Atlanta, GA [Member] | Operating Properties Office [Member]
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	368
Initial cost to company, building and improvements	0
Costs capitalized subsequent to acquisition, land and improvements	(3)
Costs capitalized subsequent to acquisition, building and improvements	21,933
Gross amount at which carried at close of period, land and improvements	365
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	21,933
Gross amount at which carried at close of period, total	22,298
Accumulated depreciation	13,235
Life on which depreciation in statement of operations is computed (b)	30 years
221 Peachtree Center Avenue Parking Garage Atlanta, GA [Member] | Operating Properties Office [Member]
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	4,217
Initial cost to company, building and improvements	13,337
Costs capitalized subsequent to acquisition, land and improvements	1
Costs capitalized subsequent to acquisition, building and improvements	347
Gross amount at which carried at close of period, land and improvements	4,218
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	13,684
Gross amount at which carried at close of period, total	17,902
Accumulated depreciation	2,755
Life on which depreciation in statement of operations is computed (b)	39 years
333 North Point Center East Suburban Atlanta, GA [Member] | Operating Properties Office [Member]
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	551
Initial cost to company, building and improvements	0
Costs capitalized subsequent to acquisition, land and improvements	1
Costs capitalized subsequent to acquisition, building and improvements	15,626
Gross amount at which carried at close of period, land and improvements	552
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	15,626
Gross amount at which carried at close of period, total	16,178
Accumulated depreciation	10,016
Life on which depreciation in statement of operations is computed (b)	30 years
100 North Point Center East Suburban Atlanta, GA [Member] | Operating Properties Office [Member]
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	1,475
Initial cost to company, building and improvements	9,625
Costs capitalized subsequent to acquisition, land and improvements	(11)
Costs capitalized subsequent to acquisition, building and improvements	3,266
Gross amount at which carried at close of period, land and improvements	1,464
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	12,891
Gross amount at which carried at close of period, total	14,355
Accumulated depreciation	7,519
Life on which depreciation in statement of operations is computed (b)	25 years
200 North Point Center East Suburban Atlanta, GA [Member] | Operating Properties Office [Member]
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	1,726
Initial cost to company, building and improvements	7,920
Costs capitalized subsequent to acquisition, land and improvements	(14)
Costs capitalized subsequent to acquisition, building and improvements	4,690
Gross amount at which carried at close of period, land and improvements	1,712
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	12,610
Gross amount at which carried at close of period, total	14,322
Accumulated depreciation	6,045
Life on which depreciation in statement of operations is computed (b)	25 years
Land Adjacent to The Avenue Forsyth Suburban Atlanta [Member] | Commercial Real Estate [Member] | Impairment moved to discontinued operations
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	11,240
Initial cost to company, building and improvements	0
Costs capitalized subsequent to acquisition, land and improvements	(7,540)
Costs capitalized subsequent to acquisition, building and improvements	0
Gross amount at which carried at close of period, land and improvements	3,700
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	0
Gross amount at which carried at close of period, total	3,700
Accumulated depreciation	0
Life on which depreciation in statement of operations is computed (b)	0 years
Blalock Lakes Suburban Atlanta, GA [Member] | Commercial Real Estate [Member] | Impairment moved to discontinued operations
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	9,646
Initial cost to company, building and improvements	0
Costs capitalized subsequent to acquisition, land and improvements	(6,301)
Costs capitalized subsequent to acquisition, building and improvements	0
Gross amount at which carried at close of period, land and improvements	3,345
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	0
Gross amount at which carried at close of period, total	3,345
Accumulated depreciation	0
Life on which depreciation in statement of operations is computed (b)	0 years
Blalock Lakes Suburban Atlanta, GA [Member] | Residential Lots [Member] | Impairment moved to discontinued operations
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	17,657
Initial cost to company, building and improvements	0
Costs capitalized subsequent to acquisition, land and improvements	(12,386)
Costs capitalized subsequent to acquisition, building and improvements	0
Gross amount at which carried at close of period, land and improvements	5,271
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	0
Gross amount at which carried at close of period, total	5,271
Accumulated depreciation	0
549 / 555 / 557 Peachtree Street Atlanta, GA [Member] | Commercial Real Estate [Member] | Impairment moved to discontinued operations
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	5,988
Initial cost to company, building and improvements	0
Costs capitalized subsequent to acquisition, land and improvements	(3,363)
Costs capitalized subsequent to acquisition, building and improvements	0
Gross amount at which carried at close of period, land and improvements	2,625
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	0
Gross amount at which carried at close of period, total	2,625
Accumulated depreciation	0
Life on which depreciation in statement of operations is computed (b)	0 years
North Point Suburban Atlanta, GA [Member] | Commercial Real Estate [Member] | Impairment moved to discontinued operations
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	10,294
Initial cost to company, building and improvements	0
Costs capitalized subsequent to acquisition, land and improvements	(8,851)
Costs capitalized subsequent to acquisition, building and improvements	0
Gross amount at which carried at close of period, land and improvements	1,443
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	0
Gross amount at which carried at close of period, total	1,443
Accumulated depreciation	0
Life on which depreciation in statement of operations is computed (b)	0 years
Wildwood Suburban Atlanta, GA [Member] | Commercial Real Estate [Member] | Impairment moved to discontinued operations
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	10,214
Initial cost to company, building and improvements	0
Costs capitalized subsequent to acquisition, land and improvements	(9,539)
Costs capitalized subsequent to acquisition, building and improvements	0
Gross amount at which carried at close of period, land and improvements	675
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	0
Gross amount at which carried at close of period, total	675
Accumulated depreciation	0
Life on which depreciation in statement of operations is computed (b)	0 years
Callaway Gardens Pine Mountain, GA [Member] | Residential Lots [Member] | Impairment moved to discontinued operations
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	1,584
Initial cost to company, building and improvements	0
Costs capitalized subsequent to acquisition, land and improvements	3,003
Costs capitalized subsequent to acquisition, building and improvements	0
Gross amount at which carried at close of period, land and improvements	4,587
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	0
Gross amount at which carried at close of period, total	4,587
Accumulated depreciation	0
Life on which depreciation in statement of operations is computed (b)	0 years
Northpark Town Center [Member] | Operating Properties Office [Member]
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	24,577
Initial cost to company, building and improvements	295,825
Costs capitalized subsequent to acquisition, land and improvements	0
Costs capitalized subsequent to acquisition, building and improvements	3,999
Gross amount at which carried at close of period, land and improvements	24,577
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	299,824
Gross amount at which carried at close of period, total	324,401
Accumulated depreciation	3,119
Life on which depreciation in statement of operations is computed (b)	39 years
Fifth Third Center [Member] | Operating Properties Office [Member]
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	22,863
Initial cost to company, building and improvements	180,430
Costs capitalized subsequent to acquisition, land and improvements	0
Costs capitalized subsequent to acquisition, building and improvements	2,107
Gross amount at which carried at close of period, land and improvements	22,863
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	182,537
Gross amount at which carried at close of period, total	205,400
Accumulated depreciation	2,445
Life on which depreciation in statement of operations is computed (b)	40 years
Operating Properties [Member]
Real Estate and Accumulated Depreciation
Encumbrances	652,144
Initial cost to company, land and improvements	529,289
Initial cost to company, building and improvements	1,701,743
Costs capitalized subsequent to acquisition, land and improvements	(37,601)
Costs capitalized subsequent to acquisition, building and improvements	426,057
Gross amount at which carried at close of period, land and improvements	491,688
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	2,127,523
Gross amount at which carried at close of period, total	2,619,488
Accumulated depreciation	324,543
Projects under Development [Member]
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	4,373
Initial cost to company, building and improvements	0
Costs capitalized subsequent to acquisition, land and improvements	0
Costs capitalized subsequent to acquisition, building and improvements	87,242
Gross amount at which carried at close of period, land and improvements	4,373
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	87,242
Gross amount at which carried at close of period, total	91,615
Accumulated depreciation	0
Projects under Development [Member] | Colorado Tower [Member]
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	0
Initial cost to company, building and improvements	0
Costs capitalized subsequent to acquisition, land and improvements	0
Costs capitalized subsequent to acquisition, building and improvements	85,843
Gross amount at which carried at close of period, land and improvements	0
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	85,843
Gross amount at which carried at close of period, total	85,843
Accumulated depreciation	0
Life on which depreciation in statement of operations is computed (b)	30 years
Projects under Development [Member] | Research Park Austin [Member]
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	4,373
Initial cost to company, building and improvements	0
Costs capitalized subsequent to acquisition, land and improvements	0
Costs capitalized subsequent to acquisition, building and improvements	1,399
Gross amount at which carried at close of period, land and improvements	4,373
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	1,399
Gross amount at which carried at close of period, total	5,772
Accumulated depreciation	$ 0
Life on which depreciation in statement of operations is computed (b)	0 years