Real Estate and Accumulated Depreciation (Schedule of Properties) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 496,651
Initial Cost to Company
Land and Improvements	330,811
Buildings and Improvements	2,854,212
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	(11,932)
Building and Improvements less Cost of Sales, Transfers and Other	720,708
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	318,879
Building and Improvements less Cost of Sales, Transfers and Other	3,574,920
Total	3,893,799	$ 3,814,986	$ 2,606,343	$ 2,619,488
Accumulated Depreciation	275,977	$ 215,856	$ 359,422	$ 324,543
OPERATING PROPERTIES
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	496,651
Initial Cost to Company
Land and Improvements	281,146
Buildings and Improvements	2,854,212
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	5,176
Building and Improvements less Cost of Sales, Transfers and Other	468,062
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	286,322
Building and Improvements less Cost of Sales, Transfers and Other	3,322,274
Total	3,608,596
Accumulated Depreciation	275,977
OPERATING PROPERTIES | Austin, TX | Colorado Tower
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	119,165
Initial Cost to Company
Land and Improvements	0
Buildings and Improvements	0
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	1,600
Building and Improvements less Cost of Sales, Transfers and Other	120,853
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	1,600
Building and Improvements less Cost of Sales, Transfers and Other	120,853
Total	122,453
Accumulated Depreciation	$ 16,599
Life on Which Depreciation in 2016 Statement of Operations is Computed	30 years
OPERATING PROPERTIES | Austin, TX | 816 Congress
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 82,742
Initial Cost to Company
Land and Improvements	6,817
Buildings and Improvements	89,891
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	3,282
Building and Improvements less Cost of Sales, Transfers and Other	18,631
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	10,099
Building and Improvements less Cost of Sales, Transfers and Other	108,522
Total	118,621
Accumulated Depreciation	$ 19,895
Life on Which Depreciation in 2016 Statement of Operations is Computed	42 years
OPERATING PROPERTIES | Austin, TX | Research Park
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	4,373
Buildings and Improvements	0
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	801
Building and Improvements less Cost of Sales, Transfers and Other	42,307
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	5,174
Building and Improvements less Cost of Sales, Transfers and Other	42,307
Total	47,481
Accumulated Depreciation	$ 2,772
Life on Which Depreciation in 2016 Statement of Operations is Computed	30 years
OPERATING PROPERTIES | Austin, TX | One Eleven Congress
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	33,841
Buildings and Improvements	201,707
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	18,682
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	33,841
Building and Improvements less Cost of Sales, Transfers and Other	220,389
Total	254,230
Accumulated Depreciation	$ 9,409
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Austin, TX | San Jacinto Center
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	34,068
Buildings and Improvements	176,535
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	(579)
Building and Improvements less Cost of Sales, Transfers and Other	(759)
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	33,489
Building and Improvements less Cost of Sales, Transfers and Other	175,776
Total	209,265
Accumulated Depreciation	$ 7,513
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Atlanta, GA | Northpark Town Center
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	22,350
Buildings and Improvements	295,825
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	47,856
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	22,350
Building and Improvements less Cost of Sales, Transfers and Other	343,681
Total	366,031
Accumulated Depreciation	$ 41,431
Life on Which Depreciation in 2016 Statement of Operations is Computed	39 years
OPERATING PROPERTIES | Atlanta, GA | Promenade
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 102,071
Initial Cost to Company
Land and Improvements	13,439
Buildings and Improvements	102,790
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	36,600
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	13,439
Building and Improvements less Cost of Sales, Transfers and Other	139,390
Total	152,829
Accumulated Depreciation	$ 40,608
Life on Which Depreciation in 2016 Statement of Operations is Computed	34 years
OPERATING PROPERTIES | Atlanta, GA | Meridian Mark Plaza
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 23,970
Initial Cost to Company
Land and Improvements	2,219
Buildings and Improvements	0
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	30,108
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	2,219
Building and Improvements less Cost of Sales, Transfers and Other	30,108
Total	32,327
Accumulated Depreciation	$ 19,782
Life on Which Depreciation in 2016 Statement of Operations is Computed	30 years
OPERATING PROPERTIES | Atlanta, GA | 3344 Peachtree
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	16,110
Buildings and Improvements	176,153
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	6,849
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	16,110
Building and Improvements less Cost of Sales, Transfers and Other	183,002
Total	199,112
Accumulated Depreciation	$ 8,811
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Atlanta, GA | One Buckhead Plaza
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	17,011
Buildings and Improvements	159,564
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	2,564
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	17,011
Building and Improvements less Cost of Sales, Transfers and Other	162,128
Total	179,139
Accumulated Depreciation	$ 7,954
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Atlanta, GA | 3350 Peachtree
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	16,836
Buildings and Improvements	108,177
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	131
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	16,836
Building and Improvements less Cost of Sales, Transfers and Other	108,308
Total	125,144
Accumulated Depreciation	$ 5,972
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Atlanta, GA | 3348 Peachtree
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	6,707
Buildings and Improvements	69,723
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	5
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	6,707
Building and Improvements less Cost of Sales, Transfers and Other	69,728
Total	76,435
Accumulated Depreciation	$ 3,804
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Atlanta, GA | 8000 Avalon
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	4,130
Buildings and Improvements	0
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	72
Building and Improvements less Cost of Sales, Transfers and Other	67,391
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	4,202
Building and Improvements less Cost of Sales, Transfers and Other	67,391
Total	71,593
Accumulated Depreciation	$ 229
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Atlanta, GA | Two Buckhead Plaza
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	18,053
Buildings and Improvements	74,547
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	1,315
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	18,053
Building and Improvements less Cost of Sales, Transfers and Other	75,862
Total	93,915
Accumulated Depreciation	$ 3,918
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Charlotte, NC | Fifth Third Center
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 145,974
Initial Cost to Company
Land and Improvements	22,591
Buildings and Improvements	180,430
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	14,669
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	22,591
Building and Improvements less Cost of Sales, Transfers and Other	195,099
Total	217,690
Accumulated Depreciation	$ 24,118
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Charlotte, NC | Hearst Tower
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	9,977
Buildings and Improvements	323,299
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	4,219
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	9,977
Building and Improvements less Cost of Sales, Transfers and Other	327,518
Total	337,495
Accumulated Depreciation	$ 15,071
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Charlotte, NC | NASCAR Plaza
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	51
Buildings and Improvements	115,238
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	2,043
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	51
Building and Improvements less Cost of Sales, Transfers and Other	117,281
Total	117,332
Accumulated Depreciation	$ 6,316
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Tampa, FL | Corporate Center
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	7,298
Buildings and Improvements	272,148
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	21,707
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	7,298
Building and Improvements less Cost of Sales, Transfers and Other	293,855
Total	301,153
Accumulated Depreciation	$ 14,042
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Tampa, FL | The Pointe
Initial Cost to Company
Land and Improvements	$ 9,404
Buildings and Improvements	54,694
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	2,368
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	9,404
Building and Improvements less Cost of Sales, Transfers and Other	57,062
Total	66,466
Accumulated Depreciation	$ 3,647
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Tampa, FL | Harborview Plaza
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	10,800
Buildings and Improvements	39,136
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	1,065
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	10,800
Building and Improvements less Cost of Sales, Transfers and Other	40,201
Total	51,001
Accumulated Depreciation	$ 2,675
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Phoenix, AZ | Hayden Ferry
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	13,102
Buildings and Improvements	262,578
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	12,397
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	13,102
Building and Improvements less Cost of Sales, Transfers and Other	274,975
Total	288,077
Accumulated Depreciation	$ 15,585
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Phoenix, AZ | 111 West Rio
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	6,076
Buildings and Improvements	56,647
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	16,217
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	6,076
Building and Improvements less Cost of Sales, Transfers and Other	72,864
Total	78,940
Accumulated Depreciation	$ 1,245
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Phoenix, AZ | Tempe Gateway
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	5,893
Buildings and Improvements	95,130
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	844
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	5,893
Building and Improvements less Cost of Sales, Transfers and Other	95,974
Total	101,867
Accumulated Depreciation	$ 4,581
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
PROJECTS UNDER DEVELOPMENT
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	28,131
Buildings and Improvements	0
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	205
Building and Improvements less Cost of Sales, Transfers and Other	252,646
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	28,336
Building and Improvements less Cost of Sales, Transfers and Other	252,646
Total	280,982
Accumulated Depreciation	0
PROJECTS UNDER DEVELOPMENT | Austin, TX | 300 Colorado
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land and Improvements	0
Buildings and Improvements	0
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	0
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	0
Total	0
Accumulated Depreciation	0
PROJECTS UNDER DEVELOPMENT | Atlanta, GA | NCR Phase 1
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land and Improvements	18,015
Buildings and Improvements	0
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	0
Building and Improvements less Cost of Sales, Transfers and Other	194,613
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	18,015
Building and Improvements less Cost of Sales, Transfers and Other	194,613
Total	212,628
Accumulated Depreciation	0
PROJECTS UNDER DEVELOPMENT | Atlanta, GA | NCR Phase II
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land and Improvements	10,116
Buildings and Improvements	0
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	205
Building and Improvements less Cost of Sales, Transfers and Other	58,033
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	10,321
Building and Improvements less Cost of Sales, Transfers and Other	58,033
Total	68,354
Accumulated Depreciation	0
LAND
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land and Improvements	21,534
Buildings and Improvements	0
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	(17,313)
Building and Improvements less Cost of Sales, Transfers and Other	0
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	4,221
Building and Improvements less Cost of Sales, Transfers and Other	0
Total	4,221
Accumulated Depreciation	0
Commercial Land
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land and Improvements	21,534
Buildings and Improvements	0
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	(17,313)
Building and Improvements less Cost of Sales, Transfers and Other	0
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	4,221
Building and Improvements less Cost of Sales, Transfers and Other	0
Total	4,221
Accumulated Depreciation	0
Commercial Land | Suburban Atlanta, GA | Land Adjacent to The Avenue Forsyth
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land and Improvements	11,240
Buildings and Improvements	0
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	(7,540)
Building and Improvements less Cost of Sales, Transfers and Other	0
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	3,700
Building and Improvements less Cost of Sales, Transfers and Other	0
Total	3,700
Accumulated Depreciation	0
Commercial Land | Suburban Atlanta, GA | North Point
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land and Improvements	10,294
Buildings and Improvements	0
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	(9,773)
Building and Improvements less Cost of Sales, Transfers and Other	0
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	521
Building and Improvements less Cost of Sales, Transfers and Other	0
Total	521
Accumulated Depreciation	$ 0