Real Estate and Accumulated Depreciation (Schedule of Properties) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Real Estate and Accumulated Depreciation [Line Items]
Document Period End Date	Dec. 31, 2015
Real Estate and Accumulated Depreciation
Gross amount at which carried at close of period, total	$ 2,606,343	$ 2,619,488	$ 2,164,815	$ 997,323
Accumulated depreciation	359,422	$ 324,543	$ 257,151	$ 258,258
Commercial Real Estate [Member] | Impairment moved to discontinued operations
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	29,633
Initial cost to company, building and improvements	0
Costs capitalized subsequent to acquisition, land and improvements	(16,391)
Costs capitalized subsequent to acquisition, building and improvements	0
Gross amount at which carried at close of period, land and improvements	13,242
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	13,242
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	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
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	76,561
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	672,108
Gross amount at which carried at close of period, total	945,759
Accumulated depreciation	$ 85,617
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	98,643
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	239,655
Gross amount at which carried at close of period, total	249,044
Accumulated depreciation	$ 88,637
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	$ 181,770
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	36,047
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	165,394
Gross amount at which carried at close of period, total	252,658
Accumulated depreciation	$ 26,333
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	$ 108,203
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	33,987
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	136,777
Gross amount at which carried at close of period, total	150,216
Accumulated depreciation	$ 25,841
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	$ 129,342
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	31,953
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	99,323
Gross amount at which carried at close of period, total	104,549
Accumulated depreciation	$ 68,082
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	10,712
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	100,603
Gross amount at which carried at close of period, total	110,702
Accumulated depreciation	$ 10,096
Life on which depreciation in statement of operations is computed (b)	42 years
Meridian Mark Plaza Atlanta, GA [Member] | Operating Properties Office [Member]
Real Estate and Accumulated Depreciation
Encumbrances	$ 24,978
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	28,142
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	28,142
Gross amount at which carried at close of period, total	30,361
Accumulated depreciation	$ 17,656
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	$ 3,127
Life on which depreciation in statement of operations is computed (b)	39 years
100 North Point Center East Suburban Atlanta, GA [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	2,404
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	12,029
Gross amount at which carried at close of period, total	13,493
Accumulated depreciation	$ 7,072
Life on which depreciation in statement of operations is computed (b)	25 years
Research Park Austin [Member] | Operating Properties Office [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	26,510
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	26,510
Gross amount at which carried at close of period, total	30,883
Accumulated depreciation	$ 0
Life on which depreciation in statement of operations is computed (b)	0 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
Colorado Tower [Member] | Commercial Real Estate [Member] | Impairment moved to discontinued operations
Real Estate and Accumulated Depreciation
Encumbrances	$ 0
Initial cost to company, land and improvements	8,099
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	0
Gross amount at which carried at close of period, land and improvements	8,099
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	8,099
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
Callaway Gardens Pine Mountain [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	10,741
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	306,566
Gross amount at which carried at close of period, total	331,143
Accumulated depreciation	$ 15,006
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	5,701
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	186,131
Gross amount at which carried at close of period, total	208,994
Accumulated depreciation	$ 8,671
Life on which depreciation in statement of operations is computed (b)	40 years
Colorado Tower [Member] | Operating Properties Office [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	4,014
Costs capitalized subsequent to acquisition, building and improvements	110,906
Gross amount at which carried at close of period, land and improvements	4,014
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	110,906
Gross amount at which carried at close of period, total	114,920
Accumulated depreciation	$ 3,284
Life on which depreciation in statement of operations is computed (b)	30 years
Projects under Development [Member]
Real Estate and Accumulated Depreciation
Encumbrances	$ 0
Initial cost to company, land and improvements	20,032
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	7,858
Gross amount at which carried at close of period, land and improvements	20,032
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	7,858
Gross amount at which carried at close of period, total	27,890
Accumulated depreciation	0
Projects under Development [Member] | Colorado Tower [Member]
Real Estate and Accumulated Depreciation
Encumbrances	0
Initial cost to company, land and improvements	20,032
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	7,858
Gross amount at which carried at close of period, land and improvements	20,032
Gross amount at which carried at close of period, building and Improvements less cost of sales, transfers and other	7,858
Gross amount at which carried at close of period, total	27,890
Accumulated depreciation	$ 0
Life on which depreciation in statement of operations is computed (b)	0 years