Real Estate and Accumulated Depreciation (Schedule of Properties) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Initial Cost to Company
Land and Improvements	$ 405,065
Buildings and Improvements	2,854,212
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	(11,804)
Building and Improvements less Cost of Sales, Transfers and Other	873,813
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	393,261
Building and Improvements less Cost of Sales, Transfers and Other	3,728,025
Total	4,121,286	$ 3,893,799	$ 3,814,986	$ 2,606,343
Accumulated Depreciation	421,495	$ 275,977	$ 215,856	$ 359,422
OPERATING PROPERTIES
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	467,362
Initial Cost to Company
Land and Improvements	309,277
Buildings and Improvements	2,854,212
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	5,602
Building and Improvements less Cost of Sales, Transfers and Other	855,415
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	314,879
Building and Improvements less Cost of Sales, Transfers and Other	3,709,627
Total	4,024,506
Accumulated Depreciation	421,495
OPERATING PROPERTIES | Austin, TX | Colorado Tower
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	119,427
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	$ 23,757
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	$ 81,676
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	23,333
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	113,224
Total	123,323
Accumulated Depreciation	$ 25,159
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	42,390
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	42,390
Total	47,564
Accumulated Depreciation	$ 5,282
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	25,218
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	226,925
Total	260,766
Accumulated Depreciation	$ 17,823
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	8,180
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	184,715
Total	218,204
Accumulated Depreciation	$ 13,645
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	59,539
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	355,364
Total	377,714
Accumulated Depreciation	$ 54,804
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	$ 99,238
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	40,193
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	142,983
Total	156,422
Accumulated Depreciation	$ 47,862
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	$ 23,524
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	29,835
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	29,835
Total	32,054
Accumulated Depreciation	$ 20,836
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	11,798
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	187,951
Total	204,061
Accumulated Depreciation	$ 16,117
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Atlanta, GA | Buckhead Plaza
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company
Land and Improvements	35,064
Buildings and Improvements	234,111
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	6,066
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	35,064
Building and Improvements less Cost of Sales, Transfers and Other	240,177
Total	275,241
Accumulated Depreciation	$ 20,965
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	2,623
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	110,800
Total	127,636
Accumulated Depreciation	$ 9,979
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	701
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	70,424
Total	77,131
Accumulated Depreciation	$ 6,706
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	72,747
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	72,747
Total	76,949
Accumulated Depreciation	$ 3,245
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Atlanta, GA | Spring & 8th
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	426
Building and Improvements less Cost of Sales, Transfers and Other	301,249
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	28,557
Building and Improvements less Cost of Sales, Transfers and Other	301,249
Total	329,806
Accumulated Depreciation	$ 8,094
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Charlotte, NC | Fifth Third Center
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	18,449
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	198,879
Total	221,470
Accumulated Depreciation	$ 32,760
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	5,108
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	328,407
Total	338,384
Accumulated Depreciation	$ 26,928
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	2,389
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	117,627
Total	117,678
Accumulated Depreciation	$ 10,959
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	37,671
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	309,819
Total	317,117
Accumulated Depreciation	$ 26,604
Life on Which Depreciation in 2016 Statement of Operations is Computed	40 years
OPERATING PROPERTIES | Tampa, FL | The Pointe
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
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	3,807
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	58,501
Total	67,905
Accumulated Depreciation	$ 6,286
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	3,256
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	42,392
Total	53,192
Accumulated Depreciation	$ 4,257
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	19,541
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	282,119
Total	295,221
Accumulated Depreciation	$ 26,820
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	16,748
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	73,395
Total	79,471
Accumulated Depreciation	$ 4,078
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	3,721
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	98,851
Total	104,744
Accumulated Depreciation	$ 8,529
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	5,819
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	18,398
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	5,819
Building and Improvements less Cost of Sales, Transfers and Other	18,398
Total	24,217
Accumulated Depreciation	0
PROJECTS UNDER DEVELOPMENT | Suburban Atlanta, GA | 10000 Avalon
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land and Improvements	5,819
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	18,398
Gross Amount at Which Carried at Close of Period
Land and Improvements less Cost of Sales, Transfers and Other	5,819
Building and Improvements less Cost of Sales, Transfers and Other	18,398
Total	24,217
Accumulated Depreciation	0
LAND
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land and Improvements	89,969
Buildings and Improvements	0
Costs Capitalized Subsequent to Acquisition
Land and Improvements less Cost of Sales, Transfers and Other	(17,406)
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	72,563
Building and Improvements less Cost of Sales, Transfers and Other	0
Total	72,563
Accumulated Depreciation	0
LAND | Atlanta, GA | 901 West Peachtree St.
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land and Improvements	11,883
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	11,883
Building and Improvements less Cost of Sales, Transfers and Other	0
Total	11,883
Accumulated Depreciation	0
LAND | Atlanta, GA | 3rd and West Peachtree St.
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land and Improvements	37,037
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	37,037
Building and Improvements less Cost of Sales, Transfers and Other	0
Total	37,037
Accumulated Depreciation	0
LAND | Phoenix, AZ | 100 Mill
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company
Land and Improvements	19,515
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	19,515
Building and Improvements less Cost of Sales, Transfers and Other	0
Total	19,515
Accumulated Depreciation	0
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
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	(9,866)
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	428
Building and Improvements less Cost of Sales, Transfers and Other	0
Total	428
Accumulated Depreciation	$ 0