SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Total revenue	$ 486,819	$ 534,852	$ 580,417
Rents
Disaggregation of Revenue [Line Items]
Total revenue	451,519	513,504	554,665
Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	433,766	497,264	525,748
Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	343,924	393,178	411,524
Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	89,842	104,086	114,224
Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	5,377	4,868	8,857
Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	12,376	11,372	20,060
Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	26,444	18,580	19,626
Other
Disaggregation of Revenue [Line Items]
Total revenue	8,856	2,768	6,126
Philadelphia CBD
Disaggregation of Revenue [Line Items]
Total revenue	207,858	232,028	263,769
Philadelphia CBD | Rents
Disaggregation of Revenue [Line Items]
Total revenue	204,848	229,619	259,471
Philadelphia CBD | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	191,026	217,696	237,061
Philadelphia CBD | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	149,441	166,286	178,481
Philadelphia CBD | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	41,585	51,410	58,580
Philadelphia CBD | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	2,064	1,146	3,745
Philadelphia CBD | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	11,758	10,777	18,665
Philadelphia CBD | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	893	927	876
Philadelphia CBD | Other
Disaggregation of Revenue [Line Items]
Total revenue	2,117	1,482	3,422
Pennsylvania Suburbs
Disaggregation of Revenue [Line Items]
Total revenue	124,566	141,613	141,084
Pennsylvania Suburbs | Rents
Disaggregation of Revenue [Line Items]
Total revenue	124,256	141,162	140,413
Pennsylvania Suburbs | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	124,106	140,995	140,251
Pennsylvania Suburbs | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	113,748	128,044	125,969
Pennsylvania Suburbs | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	10,358	12,951	14,282
Pennsylvania Suburbs | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	(9)	(12)	(12)
Pennsylvania Suburbs | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	159	179	174
Pennsylvania Suburbs | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	34	39	43
Pennsylvania Suburbs | Other
Disaggregation of Revenue [Line Items]
Total revenue	276	412	628
Austin, Texas
Disaggregation of Revenue [Line Items]
Total revenue	101,680	102,982	104,157
Austin, Texas | Rents
Disaggregation of Revenue [Line Items]
Total revenue	100,826	102,069	101,783
Austin, Texas | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	97,395	98,489	96,980
Austin, Texas | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	62,545	63,366	62,232
Austin, Texas | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	34,850	35,123	34,748
Austin, Texas | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	3,322	3,531	4,638
Austin, Texas | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	109	49	165
Austin, Texas | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	452	689	1,956
Austin, Texas | Other
Disaggregation of Revenue [Line Items]
Total revenue	402	224	418
Metropolitan Washington, D.C.
Disaggregation of Revenue [Line Items]
Total revenue	19,865	40,223	51,498
Metropolitan Washington, D.C. | Rents
Disaggregation of Revenue [Line Items]
Total revenue	13,173	33,509	44,273
Metropolitan Washington, D.C. | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	13,056	33,374	43,449
Metropolitan Washington, D.C. | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	12,410	29,830	39,420
Metropolitan Washington, D.C. | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	646	3,544	4,029
Metropolitan Washington, D.C. | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0
Metropolitan Washington, D.C. | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	117	135	824
Metropolitan Washington, D.C. | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	6,548	6,541	6,922
Metropolitan Washington, D.C. | Other
Disaggregation of Revenue [Line Items]
Total revenue	144	173	303
Other
Disaggregation of Revenue [Line Items]
Total revenue	14,015	13,469	14,558
Other | Rents
Disaggregation of Revenue [Line Items]
Total revenue	10,913	10,900	11,632
Other | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	10,680	10,465	10,914
Other | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	8,020	8,064	7,834
Other | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	2,660	2,401	3,080
Other | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	0	203	486
Other | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	233	232	232
Other | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	3,077	2,560	2,915
Other | Other
Disaggregation of Revenue [Line Items]
Total revenue	25	9	11
Corporate
Disaggregation of Revenue [Line Items]
Total revenue	18,835	4,537	5,351
Corporate | Rents
Disaggregation of Revenue [Line Items]
Total revenue	(2,497)	(3,755)	(2,907)
Corporate | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	(2,497)	(3,755)	(2,907)
Corporate | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	(2,240)	(2,412)	(2,412)
Corporate | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	(257)	(1,343)	(495)
Corporate | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0
Corporate | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	0	0	0
Corporate | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	15,440	7,824	6,914
Corporate | Other
Disaggregation of Revenue [Line Items]
Total revenue	$ 5,892	$ 468	$ 1,344