Investment in Unconsolidated Joint Ventures - (Summary of Financial Position and Operations) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
SUMMARY OF FINANCIAL POSITION:
Total Assets	$ 6,886,596		$ 6,886,596		$ 7,151,447
Total Equity	4,499,812		4,499,812		4,359,274
SUMMARY OF OPERATIONS:
Total Revenues	183,380	$ 188,323	557,193	$ 463,076
Net Income (Loss)	28,115	$ 20,374	226,159	33,306
Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF FINANCIAL POSITION:
Total Assets	592,529		592,529		658,303
Total Debt	214,514		214,514		165,039
Total Equity	250,143		250,143		354,404
Company’s Investment	111,937		111,937		114,158
SUMMARY OF OPERATIONS:
Total Revenues			45,041	85,029
Net Income (Loss)			12,002	19,959
Company's Share of Income (Loss)			6,751	9,779
DC Charlotte Plaza LLLP | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF FINANCIAL POSITION:
Total Assets	175,827		175,827		179,694
Total Debt	0		0		0
Total Equity	90,767		90,767		90,373
Company’s Investment	48,040		48,040		48,058
SUMMARY OF OPERATIONS:
Total Revenues			15,476	10,716
Net Income (Loss)			5,597	4,196
Company's Share of Income (Loss)			2,583	2,098
Austin 300 Colorado Project, LP | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF FINANCIAL POSITION:
Total Assets	165,704		165,704		112,630
Total Debt	71,681		71,681		21,430
Total Equity	68,191		68,191		68,101
Company’s Investment	37,929		37,929		36,846
SUMMARY OF OPERATIONS:
Total Revenues			264	319
Net Income (Loss)			90	152
Company's Share of Income (Loss)			45	76
AMCO 120 WT Holdings, LLC | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF FINANCIAL POSITION:
Total Assets	86,151		86,151		77,377
Total Debt	0		0		0
Total Equity	83,467		83,467		70,696
Company’s Investment	15,769		15,769		13,362
SUMMARY OF OPERATIONS:
Total Revenues			1,564	5
Net Income (Loss)			(2,106)	(81)
Company's Share of Income (Loss)			(383)	0
Carolina Square Holdings LP | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF FINANCIAL POSITION:
Total Assets	111,240		111,240		114,483
Total Debt	76,108		76,108		75,662
Total Equity	22,606		22,606		25,184
Company’s Investment	13,514		13,514		14,414
SUMMARY OF OPERATIONS:
Total Revenues			10,792	8,757
Net Income (Loss)			1,936	191
Company's Share of Income (Loss)			950	5
HICO Victory Center LP | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF FINANCIAL POSITION:
Total Assets	15,835		15,835		16,045
Total Debt	0		0		0
Total Equity	15,835		15,835		15,353
Company’s Investment	10,541		10,541		10,373
SUMMARY OF OPERATIONS:
Total Revenues			241	356
Net Income (Loss)			241	356
Company's Share of Income (Loss)			121	197
Charlotte Gateway Village, LLC | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF FINANCIAL POSITION:
Total Assets	0		0		109,675
Total Debt	0		0		0
Total Equity	0		0		106,651
Company’s Investment	0		0		6,718
SUMMARY OF OPERATIONS:
Total Revenues			6,692	20,368
Net Income (Loss)			3,352	7,510
Company's Share of Income (Loss)			1,674	3,755
Wildwood Associates | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF FINANCIAL POSITION:
Total Assets	0		0		11,061
Total Debt	0		0		0
Total Equity	0		0		10,978
Company’s Investment	0		0		(521)
Crawford Long - CPI, LLC | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF FINANCIAL POSITION:
Total Assets	30,820		30,820		28,459
Total Debt	66,725		66,725		67,947
Total Equity	(37,483)		(37,483)		(40,250)
Company’s Investment	(17,877)		(17,877)		(19,205)
SUMMARY OF OPERATIONS:
Total Revenues			9,784	9,412
Net Income (Loss)			2,767	2,821
Company's Share of Income (Loss)			1,329	1,349
Terminus Office Holdings LLC | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF OPERATIONS:
Total Revenues			0	34,964
Net Income (Loss)			0	4,962
Company's Share of Income (Loss)			0	2,381
Other | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF FINANCIAL POSITION:
Total Assets	6,952		6,952		8,879
Total Debt	0		0		0
Total Equity	6,760		6,760		7,318
Company’s Investment	$ 4,021		4,021		$ 4,113
SUMMARY OF OPERATIONS:
Total Revenues			228	132
Net Income (Loss)			125	(148)
Company's Share of Income (Loss)			$ 432	$ (82)