Investment in Unconsolidated Joint Ventures - Summary of Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SUMMARY OF OPERATIONS
Total rental property revenues	$ 740,340	$ 657,515	$ 475,212
Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF OPERATIONS
Total rental property revenues	62,859	104,471	95,515
Net Income (Loss)	21,409	25,685	15,352
Company's Share of Net  Income (Loss)	7,947	12,666	12,224
Charlotte Gateway Village, LLC | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF OPERATIONS
Total rental property revenues	6,692	27,708	26,932
Net Income (Loss)	3,202	10,285	10,285
Company's Share of Net  Income (Loss)	1,658	5,143	5,143
DC Charlotte Plaza LLLP | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF OPERATIONS
Total rental property revenues	20,439	15,636	0
Net Income (Loss)	7,272	5,894	0
Company's Share of Net  Income (Loss)	3,380	2,947	(1)
Terminus Office Holdings LLC | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF OPERATIONS
Total rental property revenues	0	34,964	44,429
Net Income (Loss)	0	4,962	5,506
Company's Share of Net  Income (Loss)	(23)	2,381	2,755
Crawford Long - CPI, LLC | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF OPERATIONS
Total rental property revenues	12,650	12,664	12,383
Net Income (Loss)	3,797	3,897	3,446
Company's Share of Net  Income (Loss)	1,808	1,866	1,641
HICO Victory Center LP | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF OPERATIONS
Total rental property revenues	356	513	400
Net Income (Loss)	356	513	400
Company's Share of Net  Income (Loss)	178	276	219
Carolina Square Holdings LP | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF OPERATIONS
Total rental property revenues	14,581	12,344	10,686
Net Income (Loss)	3,061	470	(169)
Company's Share of Net  Income (Loss)	1,472	133	(275)
Austin 300 Colorado Project, LP | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF OPERATIONS
Total rental property revenues	841	422	487
Net Income (Loss)	466	199	220
Company's Share of Net  Income (Loss)	233	100	110
Wildwood Associates | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF OPERATIONS
Total rental property revenues	0	0	0
Net Income (Loss)	(9)	(100)	(1,140)
Company's Share of Net  Income (Loss)	14	(50)	2,723
AMCO 120 WT Holdings, LLC | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF OPERATIONS
Total rental property revenues	3,000	40	0
Net Income (Loss)	2,740	(341)	38
Company's Share of Net  Income (Loss)	(552)	(68)	0
Other | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF OPERATIONS
Total rental property revenues	4,300	180	198
Net Income (Loss)	524	(94)	(3,234)
Company's Share of Net  Income (Loss)	$ (221)	$ (62)	$ (91)