Investment in Unconsolidated Joint Ventures (Summary of Financial Position and Operations) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
SUMMARY OF FINANCIAL POSITION:
Total Assets	$ 909,523		$ 779,204
Total Debt	344,639		342,892
Total Equity	405,764		338,427
Company’s Investment	165,844		140,376
SUMMARY OF OPERATIONS:
Total Revenues	85,029	$ 71,194
Net Income (Loss)	19,959	14,079
Company's Share of Income (Loss)	9,779	10,173
Terminus Office Holdings LLC
SUMMARY OF FINANCIAL POSITION:
Total Assets	257,554		258,060
Total Debt	195,290		198,732
Total Equity	55,502		50,539
Company’s Investment	50,781		48,571
SUMMARY OF OPERATIONS:
Total Revenues	34,964	33,545
Net Income (Loss)	4,962	4,424
Company's Share of Income (Loss)	2,381	2,276
DC Charlotte Plaza LLLP
SUMMARY OF FINANCIAL POSITION:
Total Assets	184,544		155,530
Total Debt	0		0
Total Equity	93,775		88,922
Company’s Investment	49,759		46,554
SUMMARY OF OPERATIONS:
Total Revenues	10,716	0
Net Income (Loss)	4,196	0
Company's Share of Income (Loss)	2,098	0
Austin 300 Colorado Project, LP
SUMMARY OF FINANCIAL POSITION:
Total Assets	104,875		51,180
Total Debt	5,488		0
Total Equity	68,054		41,298
Company’s Investment	36,462		22,335
SUMMARY OF OPERATIONS:
Total Revenues	319	385
Net Income (Loss)	152	173
Company's Share of Income (Loss)	76	86
Carolina Square Holdings LP
SUMMARY OF FINANCIAL POSITION:
Total Assets	114,486		106,187
Total Debt	75,514		74,638
Total Equity	25,860		28,844
Company’s Investment	15,221		16,840
SUMMARY OF OPERATIONS:
Total Revenues	8,757	7,403
Net Income (Loss)	191	5
Company's Share of Income (Loss)	5	(173)
AMCO 120 WT Holdings, LLC
SUMMARY OF FINANCIAL POSITION:
Total Assets	70,163		36,680
Total Debt	0		0
Total Equity	62,185		31,372
Company’s Investment	11,966		5,538
SUMMARY OF OPERATIONS:
Total Revenues	5	0
Net Income (Loss)	(81)	(28)
Company's Share of Income (Loss)	0	0
HICO Victory Center LP
SUMMARY OF FINANCIAL POSITION:
Total Assets	15,382		15,069
Total Debt	0		0
Total Equity	15,196		14,801
Company’s Investment	10,264		10,003
SUMMARY OF OPERATIONS:
Total Revenues	356	282
Net Income (Loss)	356	282
Company's Share of Income (Loss)	197	160
Charlotte Gateway Village, LLC
SUMMARY OF FINANCIAL POSITION:
Total Assets	112,768		112,553
Total Debt	0		0
Total Equity	108,176		109,666
Company’s Investment	7,480		8,225
SUMMARY OF OPERATIONS:
Total Revenues	20,368	20,043
Net Income (Loss)	7,510	7,792
Company's Share of Income (Loss)	3,755	3,896
CL Realty, L.L.C.
SUMMARY OF FINANCIAL POSITION:
Total Assets	4,159		4,169
Total Debt	0		0
Total Equity	4,039		4,183
Company’s Investment	2,814		2,886
SUMMARY OF OPERATIONS:
Total Revenues	0	0
Net Income (Loss)	(129)	(116)
Company's Share of Income (Loss)	(72)	(71)
Temco Associates, LLC
SUMMARY OF FINANCIAL POSITION:
Total Assets	1,551		1,482
Total Debt	0		0
Total Equity	1,448		1,379
Company’s Investment	953		919
SUMMARY OF OPERATIONS:
Total Revenues	128	128
Net Income (Loss)	61	58
Company's Share of Income (Loss)	35	32
TR 208 Nueces Member, LLC
SUMMARY OF FINANCIAL POSITION:
Total Assets	3,176		0
Total Debt	0		0
Total Equity	1,396		0
Company’s Investment	349		0
EP II LLC
SUMMARY OF FINANCIAL POSITION:
Total Assets	247		247
Total Debt	0		0
Total Equity	165		165
Company’s Investment	28		30
SUMMARY OF OPERATIONS:
Total Revenues	1	0
Net Income (Loss)	0	(21)
Company's Share of Income (Loss)	(1)	(15)
EP I LLC
SUMMARY OF FINANCIAL POSITION:
Total Assets	461		461
Total Debt	0		0
Total Equity	296		296
Company’s Investment	2		6
SUMMARY OF OPERATIONS:
Total Revenues	3	27
Net Income (Loss)	0	1
Company's Share of Income (Loss)	(4)	(5)
Wildwood Associates
SUMMARY OF FINANCIAL POSITION:
Total Assets	11,105		11,157
Total Debt	0		0
Total Equity	10,998		11,108
Company’s Investment	(512)		(460)
SUMMARY OF OPERATIONS:
Total Revenues	0	0
Net Income (Loss)	(80)	(1,108)
Company's Share of Income (Loss)	(40)	2,739
Crawford Long - CPI, LLC
SUMMARY OF FINANCIAL POSITION:
Total Assets	29,052		26,429
Total Debt	68,347		69,522
Total Equity	(41,326)		(44,146)
Company’s Investment	(19,723)		$ (21,071)
SUMMARY OF OPERATIONS:
Total Revenues	9,412	9,381
Net Income (Loss)	2,821	2,631
Company's Share of Income (Loss)	1,349	1,254
Other
SUMMARY OF OPERATIONS:
Total Revenues	0	0
Net Income (Loss)	0	(14)
Company's Share of Income (Loss)	$ 0	$ (6)