Investment in Unconsolidated Joint Ventures - (Summary of Financial Position and Operations) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
SUMMARY OF FINANCIAL POSITION:
Total Assets	$ 6,908,448		$ 6,908,448		$ 7,151,447
Total Equity	4,515,336		4,515,336		4,359,274
SUMMARY OF OPERATIONS:
Total Revenues	179,915	$ 142,020	373,813	$ 274,753
Net Income (Loss)	23,101	$ (22,409)	198,044	12,932
Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF FINANCIAL POSITION:
Total Assets	573,607		573,607		658,303
Total Debt	197,508		197,508		165,039
Total Equity	245,989		245,989		354,404
Company’s Investment	111,504		111,504		114,158
SUMMARY OF OPERATIONS:
Total Revenues			31,995	56,338
Net Income (Loss)			9,276	13,256
Company's Share of Income (Loss)			5,140	6,538
DC Charlotte Plaza LLLP | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF FINANCIAL POSITION:
Total Assets	177,003		177,003		179,694
Total Debt	0		0		0
Total Equity	91,115		91,115		90,373
Company’s Investment	48,249		48,249		48,058
SUMMARY OF OPERATIONS:
Total Revenues			10,429	5,410
Net Income (Loss)			3,681	2,201
Company's Share of Income (Loss)			1,661	1,101
Austin 300 Colorado Project, LP | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF FINANCIAL POSITION:
Total Assets	149,576		149,576		112,630
Total Debt	54,074		54,074		21,430
Total Equity	68,196		68,196		68,101
Company’s Investment	37,648		37,648		36,846
SUMMARY OF OPERATIONS:
Total Revenues			195	222
Net Income (Loss)			95	110
Company's Share of Income (Loss)			48	55
AMCO 120 WT Holdings, LLC | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF FINANCIAL POSITION:
Total Assets	81,484		81,484		77,377
Total Debt	0		0		0
Total Equity	78,304		78,304		70,696
Company’s Investment	14,753		14,753		13,362
SUMMARY OF OPERATIONS:
Total Revenues			584	0
Net Income (Loss)			(1,310)	(32)
Company's Share of Income (Loss)			(272)	0
Carolina Square Holdings LP | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF FINANCIAL POSITION:
Total Assets	112,052		112,052		114,483
Total Debt	76,298		76,298		75,662
Total Equity	23,815		23,815		25,184
Company’s Investment	14,130		14,130		14,414
SUMMARY OF OPERATIONS:
Total Revenues			7,209	6,491
Net Income (Loss)			1,228	158
Company's Share of Income (Loss)			569	110
HICO Victory Center LP | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF FINANCIAL POSITION:
Total Assets	15,696		15,696		16,045
Total Debt	0		0		0
Total Equity	15,550		15,550		15,353
Company’s Investment	10,519		10,519		10,373
SUMMARY OF OPERATIONS:
Total Revenues			197	235
Net Income (Loss)			197	235
Company's Share of Income (Loss)			98	137
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			6,684	13,611
Net Income (Loss)			3,400	5,049
Company's Share of Income (Loss)			1,699	2,524
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	30,130		30,130		28,459
Total Debt	67,136		67,136		67,947
Total Equity	(38,463)		(38,463)		(40,250)
Company’s Investment	(18,353)		(18,353)		(19,205)
SUMMARY OF OPERATIONS:
Total Revenues			6,453	6,255
Net Income (Loss)			1,787	1,809
Company's Share of Income (Loss)			852	863
Terminus Office Holdings LLC | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF OPERATIONS:
Total Revenues			0	24,050
Net Income (Loss)			0	3,819
Company's Share of Income (Loss)			0	1,800
Other | Equity Method Investment, Nonconsolidated Investee or Group of Investees
SUMMARY OF FINANCIAL POSITION:
Total Assets	7,666		7,666		8,879
Total Debt	0		0		0
Total Equity	7,472		7,472		7,318
Company’s Investment	$ 4,558		4,558		$ 4,113
SUMMARY OF OPERATIONS:
Total Revenues			244	64
Net Income (Loss)			198	(93)
Company's Share of Income (Loss)			$ 485	$ (52)