Investment in Unconsolidated Joint Ventures (Summary of Operations) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SUMMARY OF OPERATIONS
Total Revenues	$ 104,471	$ 95,515	$ 110,664
Net Income (Loss)	25,685	15,352	77,944
Company's Share of Net Income (Loss)	12,666	12,224	47,115
Charlotte Gateway Village, LLC
SUMMARY OF OPERATIONS
Total Revenues	27,708	26,932	26,465
Net Income (Loss)	10,285	10,285	9,528
Company's Share of Net Income (Loss)	5,143	5,143	4,764
DC Charlotte Plaza LLLP
SUMMARY OF OPERATIONS
Total Revenues	15,636	0	2
Net Income (Loss)	5,894	0	2
Company's Share of Net Income (Loss)	2,947	(1)	1
Terminus Office Holdings LLC
SUMMARY OF OPERATIONS
Total Revenues	34,964	44,429	43,959
Net Income (Loss)	4,962	5,506	6,307
Company's Share of Net Income (Loss)	2,381	2,755	3,153
Crawford Long - CPI, LLC
SUMMARY OF OPERATIONS
Total Revenues	12,664	12,383	12,079
Net Income (Loss)	3,897	3,446	3,171
Company's Share of Net Income (Loss)	1,866	1,641	1,572
HICO Victory Center LP
SUMMARY OF OPERATIONS
Total Revenues	513	400	429
Net Income (Loss)	513	400	431
Company's Share of Net Income (Loss)	276	219	225
Carolina Square Holdings LP
SUMMARY OF OPERATIONS
Total Revenues	12,344	10,686	2,701
Net Income (Loss)	470	(169)	(532)
Company's Share of Net Income (Loss)	133	(275)	522
Austin 300 Colorado Project, LP
SUMMARY OF OPERATIONS
Total Revenues	422	487	0
Net Income (Loss)	199	220	0
Company's Share of Net Income (Loss)	100	110	0
Wildwood Associates
SUMMARY OF OPERATIONS
Total Revenues	0	0	0
Net Income (Loss)	(100)	(1,140)	(116)
Company's Share of Net Income (Loss)	(50)	2,723	(58)
AMCO 120 WT Holdings, LLC
SUMMARY OF OPERATIONS
Total Revenues	40	0	0
Net Income (Loss)	(341)	38	58
Company's Share of Net Income (Loss)	(68)	0	0
Other
SUMMARY OF OPERATIONS
Total Revenues	180	198	25,029
Net Income (Loss)	(94)	(3,234)	59,095
Company's Share of Net Income (Loss)	$ (62)	$ (91)	$ 36,936