Schedule III - Real Estate Assets and Accumulated Depreciation and Amortization (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	$ 145,050
Building and Improvements	302,071
In-place lease value intangible	100,790
Total	547,911
Post-acquisition Improvements	46,658
Gross Carrying Amount
Land	145,050
Building and Improvements	348,729
In-place lease value intangible	100,790
Total	594,569	$ 583,112	$ 259,962
Accumulated Depreciation & Amortization	(117,652)
Net Book Carrying Value	476,917
Encumbrances	43,392
Garden Oaks
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	1,770
Building and Improvements	17,969
In-place lease value intangible	1,021
Total	20,760
Post-acquisition Improvements	458
Gross Carrying Amount
Land	1,770
Building and Improvements	18,427
In-place lease value intangible	1,021
Total	21,218
Accumulated Depreciation & Amortization	(994)
Net Book Carrying Value	20,225
Encumbrances
Quitman
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	3,130
Building and Improvements	2,389
In-place lease value intangible	351
Total	5,870
Post-acquisition Improvements	130
Gross Carrying Amount
Land	3,130
Building and Improvements	2,519
In-place lease value intangible	351
Total	6,000
Accumulated Depreciation & Amortization	(174)
Net Book Carrying Value	5,826
Encumbrances
Chelsea Square
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	1,570
Building and Improvements	5,046
In-place lease value intangible	494
Total	7,110
Post-acquisition Improvements	170
Gross Carrying Amount
Land	1,570
Building and Improvements	5,216
In-place lease value intangible	494
Total	7,280
Accumulated Depreciation & Amortization	(357)
Net Book Carrying Value	6,923
Encumbrances
Mission Centre
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	2,020
Building and Improvements	7,690
In-place lease value intangible	890
Total	10,600
Post-acquisition Improvements	51
Gross Carrying Amount
Land	2,020
Building and Improvements	7,741
In-place lease value intangible	890
Total	10,651
Accumulated Depreciation & Amortization	(632)
Net Book Carrying Value	10,019
Encumbrances
Regency
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	960
Building and Improvements	819
In-place lease value intangible	851
Total	2,630
Post-acquisition Improvements	338
Gross Carrying Amount
Land	960
Building and Improvements	1,157
In-place lease value intangible	852
Total	2,969
Accumulated Depreciation & Amortization	(342)
Net Book Carrying Value	2,627
Encumbrances
Spring Valley
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	3,490
Building and Improvements	1,064
In-place lease value intangible	1,066
Total	5,620
Post-acquisition Improvements	997
Gross Carrying Amount
Land	3,490
Building and Improvements	2,061
In-place lease value intangible	1,066
Total	6,617
Accumulated Depreciation & Amortization	(616)
Net Book Carrying Value	6,001
Encumbrances
Northeast Square
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	1,300
Building and Improvements	3,330
In-place lease value intangible	280
Total	4,910
Post-acquisition Improvements	75
Gross Carrying Amount
Land	1,300
Building and Improvements	3,405
In-place lease value intangible	280
Total	4,985
Accumulated Depreciation & Amortization	(227)
Net Book Carrying Value	4,758
Encumbrances
One Mason
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	2,440
Building and Improvements	9,290
In-place lease value intangible	1,130
Total	12,860
Post-acquisition Improvements	30
Gross Carrying Amount
Land	2,440
Building and Improvements	9,320
In-place lease value intangible	1,130
Total	12,890
Accumulated Depreciation & Amortization	(719)
Net Book Carrying Value	12,170
Encumbrances
Tower
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	2,750
Building and Improvements	2,584
In-place lease value intangible	1,336
Total	6,670
Post-acquisition Improvements	394
Gross Carrying Amount
Land	2,750
Building and Improvements	2,977
In-place lease value intangible	1,335
Total	7,062
Accumulated Depreciation & Amortization	(584)
Net Book Carrying Value	6,478
Encumbrances
Preserve
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	9,730
Building and Improvements	9,085
In-place lease value intangible	3,485
Total	22,300
Post-acquisition Improvements	441
Gross Carrying Amount
Land	9,730
Building and Improvements	9,526
In-place lease value intangible	3,485
Total	22,740
Accumulated Depreciation & Amortization	(1,617)
Net Book Carrying Value	21,124
Encumbrances
Westheimer
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	3,800
Building and Improvements	12,416
In-place lease value intangible	2,284
Total	18,500
Post-acquisition Improvements	543
Gross Carrying Amount
Land	3,800
Building and Improvements	12,959
In-place lease value intangible	2,284
Total	19,043
Accumulated Depreciation & Amortization	(1,312)
Net Book Carrying Value	17,731
Encumbrances
Walzem Plaza
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	3,900
Building and Improvements	10,660
In-place lease value intangible	1,840
Total	16,400
Post-acquisition Improvements	1,083
Gross Carrying Amount
Land	3,900
Building and Improvements	11,743
In-place lease value intangible	1,840
Total	17,482
Accumulated Depreciation & Amortization	(1,210)
Net Book Carrying Value	16,273
Encumbrances
11811 North Freeway
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	1,980
Building and Improvements	1,037
In-place lease value intangible	2,473
Total	5,490
Post-acquisition Improvements	311
Gross Carrying Amount
Land	1,980
Building and Improvements	1,348
In-place lease value intangible	2,473
Total	5,801
Accumulated Depreciation & Amortization	(883)
Net Book Carrying Value	4,918
Encumbrances
Atrium I
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	2,540
Building and Improvements	716
In-place lease value intangible	1,494
Total	4,750
Post-acquisition Improvements	524
Gross Carrying Amount
Land	2,540
Building and Improvements	1,240
In-place lease value intangible	1,494
Total	5,275
Accumulated Depreciation & Amortization	(569)
Net Book Carrying Value	4,706
Encumbrances
North Central Plaza
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	2,330
Building and Improvements	14,511
In-place lease value intangible	2,959
Total	19,800
Post-acquisition Improvements	1,087
Gross Carrying Amount
Land	2,330
Building and Improvements	15,598
In-place lease value intangible	2,959
Total	20,887
Accumulated Depreciation & Amortization	(1,544)
Net Book Carrying Value	19,342
Encumbrances
3100 Timmons
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	10,330
Building and Improvements	3,543
In-place lease value intangible	1,427
Total	15,300
Post-acquisition Improvements	718
Gross Carrying Amount
Land	10,330
Building and Improvements	4,261
In-place lease value intangible	1,428
Total	16,019
Accumulated Depreciation & Amortization	(673)
Net Book Carrying Value	15,345
Encumbrances
Central Park
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	730
Building and Improvements	2,851
In-place lease value intangible	989
Total	4,570
Post-acquisition Improvements	23
Gross Carrying Amount
Land	730
Building and Improvements	2,874
In-place lease value intangible	989
Total	4,593
Accumulated Depreciation & Amortization	(458)
Net Book Carrying Value	4,135
Encumbrances
601 Sawyer
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	3,360
Building and Improvements	12,796
In-place lease value intangible	1,144
Total	17,300
Post-acquisition Improvements	245
Gross Carrying Amount
Land	3,360
Building and Improvements	13,041
In-place lease value intangible	1,144
Total	17,545
Accumulated Depreciation & Amortization	(798)
Net Book Carrying Value	16,747
Encumbrances
Prestonwood
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	7,410
Building and Improvements	13,895
In-place lease value intangible	1,695
Total	23,000
Post-acquisition Improvements	182
Gross Carrying Amount
Land	7,410
Building and Improvements	14,076
In-place lease value intangible	1,695
Total	23,181
Accumulated Depreciation & Amortization	(1,073)
Net Book Carrying Value	22,108
Encumbrances
Harwin
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	1,960
Building and Improvements	3,041
In-place lease value intangible	279
Total	5,280
Post-acquisition Improvements	73
Gross Carrying Amount
Land	1,960
Building and Improvements	3,114
In-place lease value intangible	279
Total	5,353
Accumulated Depreciation & Amortization	(240)
Net Book Carrying Value	5,114
Encumbrances
Fondren
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	1,650
Building and Improvements	7,326
In-place lease value intangible	1,004
Total	9,980
Post-acquisition Improvements	220
Gross Carrying Amount
Land	1,650
Building and Improvements	7,546
In-place lease value intangible	1,005
Total	10,201
Accumulated Depreciation & Amortization	(774)
Net Book Carrying Value	9,427
Encumbrances
Cornerstone
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	1,110
Building and Improvements	1,620
In-place lease value intangible	920
Total	3,650
Post-acquisition Improvements	79
Gross Carrying Amount
Land	1,110
Building and Improvements	1,699
In-place lease value intangible	920
Total	3,729
Accumulated Depreciation & Amortization	(404)
Net Book Carrying Value	3,325
Encumbrances
Northchase
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	1,700
Building and Improvements	5,821
In-place lease value intangible	1,549
Total	9,070
Post-acquisition Improvements	573
Gross Carrying Amount
Land	1,700
Building and Improvements	6,394
In-place lease value intangible	1,549
Total	9,643
Accumulated Depreciation & Amortization	(783)
Net Book Carrying Value	8,860
Encumbrances
616 FM 1960
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	1,510
Building and Improvements	8,931
In-place lease value intangible	1,269
Total	11,710
Post-acquisition Improvements	239
Gross Carrying Amount
Land	1,510
Building and Improvements	9,170
In-place lease value intangible	1,269
Total	11,949
Accumulated Depreciation & Amortization	(812)
Net Book Carrying Value	11,137
Encumbrances
Gateway
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	3,510
Building and Improvements	22,182
In-place lease value intangible	3,408
Total	29,100
Post-acquisition Improvements	743
Gross Carrying Amount
Land	3,510
Building and Improvements	22,925
In-place lease value intangible	3,408
Total	29,843
Accumulated Depreciation & Amortization	(1,928)
Net Book Carrying Value	27,915
Encumbrances
Promenade
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	5,750
Building and Improvements	12,671
In-place lease value intangible	1,579
Total	20,000
Post-acquisition Improvements	153
Gross Carrying Amount
Land	5,750
Building and Improvements	12,824
In-place lease value intangible	1,578
Total	20,152
Accumulated Depreciation & Amortization	(1,060)
Net Book Carrying Value	19,092
Encumbrances
400 North Belt
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	2,538
Building and Improvements	3,800
In-place lease value intangible	3,812
Total	10,150
Post-acquisition Improvements	2,605
Gross Carrying Amount
Land	2,538
Building and Improvements	6,405
In-place lease value intangible	3,812
Total	12,755
Accumulated Depreciation & Amortization	(5,933)
Net Book Carrying Value	6,822
Encumbrances
Commerce Plaza Hillcrest
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	6,500
Building and Improvements	1,031
In-place lease value intangible	3,869
Total	11,400
Post-acquisition Improvements	2,240
Gross Carrying Amount
Land	6,500
Building and Improvements	3,271
In-place lease value intangible	3,869
Total	13,639
Accumulated Depreciation & Amortization	(5,538)
Net Book Carrying Value	8,101
Encumbrances
Corporate Park Place
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	2,375
Building and Improvements	5,215
In-place lease value intangible	1,910
Total	9,500
Post-acquisition Improvements	1,235
Gross Carrying Amount
Land	2,375
Building and Improvements	6,450
In-place lease value intangible	1,910
Total	10,736
Accumulated Depreciation & Amortization	(3,278)
Net Book Carrying Value	7,457
Encumbrances
Skymark Tower
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	2,212
Building and Improvements	4,404
In-place lease value intangible	2,230
Total	8,846
Post-acquisition Improvements	1,701
Gross Carrying Amount
Land	2,212
Building and Improvements	6,105
In-place lease value intangible	2,230
Total	10,546
Accumulated Depreciation & Amortization	(3,565)
Net Book Carrying Value	6,981
Encumbrances
Ashford Crossing
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	2,650
Building and Improvements	4,240
In-place lease value intangible	3,710
Total	10,600
Post-acquisition Improvements	2,079
Gross Carrying Amount
Land	2,650
Building and Improvements	6,319
In-place lease value intangible	3,710
Total	12,679
Accumulated Depreciation & Amortization	(5,443)
Net Book Carrying Value	7,236
Encumbrances
Energy Plaza I&II
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	4,403
Building and Improvements	6,840
In-place lease value intangible	6,367
Total	17,610
Post-acquisition Improvements	2,104
Gross Carrying Amount
Land	4,403
Building and Improvements	8,944
In-place lease value intangible	6,367
Total	19,713
Accumulated Depreciation & Amortization	(8,503)
Net Book Carrying Value	11,210
Encumbrances
Westway One
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	5,410
Building and Improvements	11,276
In-place lease value intangible	4,950
Total	21,636
Post-acquisition Improvements	340
Gross Carrying Amount
Land	5,410
Building and Improvements	11,616
In-place lease value intangible	4,951
Total	21,977
Accumulated Depreciation & Amortization	(4,692)
Net Book Carrying Value	17,285
Encumbrances
Three Forest Plaza
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	8,910
Building and Improvements	18,186
In-place lease value intangible	8,558
Total	35,654
Post-acquisition Improvements	3,300
Gross Carrying Amount
Land	8,914
Building and Improvements	21,486
In-place lease value intangible	8,557
Total	38,957
Accumulated Depreciation & Amortization	(9,771)
Net Book Carrying Value	29,186
Encumbrances
Parkway I&II
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	2,373
Building and Improvements	4,765
In-place lease value intangible	2,352
Total	9,490
Post-acquisition Improvements	2,825
Gross Carrying Amount
Land	2,373
Building and Improvements	7,590
In-place lease value intangible	2,352
Total	12,315
Accumulated Depreciation & Amortization	(5,048)
Net Book Carrying Value	7,267
Encumbrances
Gulf Plaza
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	3,488
Building and Improvements	6,005
In-place lease value intangible	4,457
Total	13,950
Post-acquisition Improvements	1,356
Gross Carrying Amount
Land	3,488
Building and Improvements	7,361
In-place lease value intangible	4,457
Total	15,306
Accumulated Depreciation & Amortization	(6,056)
Net Book Carrying Value	9,249
Encumbrances
Timbercreek
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	724
Building and Improvements	962
In-place lease value intangible	1,211
Total	2,897
Post-acquisition Improvements	619
Gross Carrying Amount
Land	724
Building and Improvements	1,581
In-place lease value intangible	1,211
Total	3,517
Accumulated Depreciation & Amortization	(1,711)
Net Book Carrying Value	1,806
Encumbrances
Copperfield
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	605
Building and Improvements	760
In-place lease value intangible	1,054
Total	2,419
Post-acquisition Improvements	520
Gross Carrying Amount
Land	605
Building and Improvements	1,280
In-place lease value intangible	1,054
Total	2,938
Accumulated Depreciation & Amortization	(1,153)
Net Book Carrying Value	1,785
Encumbrances
One Technology Center
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	4,894
Building and Improvements	8,558
In-place lease value intangible	6,123
Total	19,575
Post-acquisition Improvements	1,866
Gross Carrying Amount
Land	4,894
Building and Improvements	10,424
In-place lease value intangible	6,123
Total	21,441
Accumulated Depreciation & Amortization	(8,471)
Net Book Carrying Value	12,970
Encumbrances
Richardson Heights
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	4,788
Building and Improvements	10,890
In-place lease value intangible	3,472
Total	19,150
Post-acquisition Improvements	7,232
Gross Carrying Amount
Land	4,788
Building and Improvements	18,122
In-place lease value intangible	3,472
Total	26,382
Accumulated Depreciation & Amortization	(9,389)
Net Book Carrying Value	16,994
Encumbrances	17,259
Bent Tree Green
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	3,003
Building and Improvements	6,272
In-place lease value intangible	2,740
Total	12,015
Post-acquisition Improvements	3,351
Gross Carrying Amount
Land	3,003
Building and Improvements	9,623
In-place lease value intangible	2,740
Total	15,366
Accumulated Depreciation & Amortization	(6,067)
Net Book Carrying Value	9,299
Encumbrances	7,435
Cooper Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	2,653
Building and Improvements	5,768
In-place lease value intangible	2,192
Total	10,613
Post-acquisition Improvements	494
Gross Carrying Amount
Land	2,653
Building and Improvements	6,262
In-place lease value intangible	2,192
Total	11,107
Accumulated Depreciation & Amortization	(4,243)
Net Book Carrying Value	6,864
Encumbrances	7,435
Mitchelldale Business Park
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation, Initial Cost
Land	4,794
Building and Improvements	9,816
In-place lease value intangible	4,565
Total	19,175
Post-acquisition Improvements	2,911
Gross Carrying Amount
Land	4,794
Building and Improvements	12,727
In-place lease value intangible	4,565
Total	22,086
Accumulated Depreciation & Amortization	(8,007)
Net Book Carrying Value	14,079
Encumbrances	$ 11,263