SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenue	$ 534,852	$ 580,417	$ 544,345
Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	393,178	411,524
Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	104,086	114,224
Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	497,264	525,748
Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	4,868	8,857
Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	11,372	20,060
Rents
Disaggregation of Revenue [Line Items]
Total revenue	513,504	554,665	515,044
Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	18,580	19,626	22,557
Other
Disaggregation of Revenue [Line Items]
Total revenue	2,768	6,126	$ 6,744
Philadelphia CBD
Disaggregation of Revenue [Line Items]
Total revenue	232,028	263,769
Philadelphia CBD | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	166,286	178,481
Philadelphia CBD | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	51,410	58,580
Philadelphia CBD | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	217,696	237,061
Philadelphia CBD | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	1,146	3,745
Philadelphia CBD | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	10,777	18,665
Philadelphia CBD | Rents
Disaggregation of Revenue [Line Items]
Total revenue	229,619	259,471
Philadelphia CBD | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	927	876
Philadelphia CBD | Other
Disaggregation of Revenue [Line Items]
Total revenue	1,482	3,422
Pennsylvania Suburbs
Disaggregation of Revenue [Line Items]
Total revenue	141,613	141,084
Pennsylvania Suburbs | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	128,044	125,969
Pennsylvania Suburbs | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	12,951	14,282
Pennsylvania Suburbs | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	140,995	140,251
Pennsylvania Suburbs | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	(12)	(12)
Pennsylvania Suburbs | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	179	174
Pennsylvania Suburbs | Rents
Disaggregation of Revenue [Line Items]
Total revenue	141,162	140,413
Pennsylvania Suburbs | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	39	43
Pennsylvania Suburbs | Other
Disaggregation of Revenue [Line Items]
Total revenue	412	628
Austin, Texas
Disaggregation of Revenue [Line Items]
Total revenue	102,982	104,157
Austin, Texas | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	63,366	62,232
Austin, Texas | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	35,123	34,748
Austin, Texas | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	98,489	96,980
Austin, Texas | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	3,531	4,638
Austin, Texas | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	49	165
Austin, Texas | Rents
Disaggregation of Revenue [Line Items]
Total revenue	102,069	101,783
Austin, Texas | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	689	1,956
Austin, Texas | Other
Disaggregation of Revenue [Line Items]
Total revenue	224	418
Metropolitan DC
Disaggregation of Revenue [Line Items]
Total revenue	40,223	51,498
Metropolitan DC | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	29,830	39,420
Metropolitan DC | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	3,544	4,029
Metropolitan DC | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	33,374	43,449
Metropolitan DC | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	0	0
Metropolitan DC | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	135	824
Metropolitan DC | Rents
Disaggregation of Revenue [Line Items]
Total revenue	33,509	44,273
Metropolitan DC | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	6,541	6,922
Metropolitan DC | Other
Disaggregation of Revenue [Line Items]
Total revenue	173	303
Other
Disaggregation of Revenue [Line Items]
Total revenue	13,469	14,558
Other | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	8,064	7,834
Other | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	2,401	3,080
Other | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	10,465	10,914
Other | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	203	486
Other | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	232	232
Other | Rents
Disaggregation of Revenue [Line Items]
Total revenue	10,900	11,632
Other | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	2,560	2,915
Other | Other
Disaggregation of Revenue [Line Items]
Total revenue	9	11
Corporate
Disaggregation of Revenue [Line Items]
Total revenue	4,537	5,351
Corporate | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	(2,412)	(2,412)
Corporate | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	(1,343)	(495)
Corporate | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	(3,755)	(2,907)
Corporate | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	0	0
Corporate | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	0	0
Corporate | Rents
Disaggregation of Revenue [Line Items]
Total revenue	(3,755)	(2,907)
Corporate | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	7,824	6,914
Corporate | Other
Disaggregation of Revenue [Line Items]
Total revenue	$ 468	$ 1,344