SEGMENT INFORMATION - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, equity method	$ 120,294	$ 169,100	$ 194,621
Equity in loss of Real Estate Ventures	(9,922)	(15,231)	(8,306)
Philadelphia CBD
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, equity method	17,524	19,897	39,939
Equity in loss of Real Estate Ventures	328	(105)	255
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, equity method	0	0	3,503
Equity in loss of Real Estate Ventures	0	0	631
Metropolitan DC
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, equity method	102,840	136,142	119,817
Equity in loss of Real Estate Ventures	(4,234)	(15,065)	(5,044)
MAP Venture
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, equity method	(70)	11,173	15,450
Equity in loss of Real Estate Ventures	(6,102)	(2,155)	(3,443)
Other
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, equity method	0	1,888	1,939
Equity in loss of Real Estate Ventures	86	407	285
Austin, Texas
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, equity method	0	0	13,973
Equity in loss of Real Estate Ventures	$ 0	$ 1,687	$ (990)