SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Total revenue	$ 506,100	$ 486,819	$ 534,852
Rents
Disaggregation of Revenue [Line Items]
Total revenue	470,851	451,519	513,504
Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	453,198	433,766	497,264
Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	353,387	343,924	393,178
Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	99,811	89,842	104,086
Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	2,582	5,377	4,868
Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	15,071	12,376	11,372
Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	24,132	26,444	18,580
Other income
Disaggregation of Revenue [Line Items]
Total revenue	11,117	8,856	2,768
Philadelphia CBD
Disaggregation of Revenue [Line Items]
Total revenue	220,875	207,858	232,028
Philadelphia CBD | Rents
Disaggregation of Revenue [Line Items]
Total revenue	218,078	204,848	229,619
Philadelphia CBD | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	202,380	191,026	217,696
Philadelphia CBD | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	151,034	149,441	166,286
Philadelphia CBD | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	51,346	41,585	51,410
Philadelphia CBD | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	1,308	2,064	1,146
Philadelphia CBD | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	14,390	11,758	10,777
Philadelphia CBD | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	287	893	927
Philadelphia CBD | Other income
Disaggregation of Revenue [Line Items]
Total revenue	2,510	2,117	1,482
Pennsylvania Suburbs
Disaggregation of Revenue [Line Items]
Total revenue	128,940	124,566	141,613
Pennsylvania Suburbs | Rents
Disaggregation of Revenue [Line Items]
Total revenue	128,551	124,256	141,162
Pennsylvania Suburbs | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	128,541	124,106	140,995
Pennsylvania Suburbs | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	116,926	113,748	128,044
Pennsylvania Suburbs | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	11,615	10,358	12,951
Pennsylvania Suburbs | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	10	(9)	(12)
Pennsylvania Suburbs | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	0	159	179
Pennsylvania Suburbs | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	35	34	39
Pennsylvania Suburbs | Other income
Disaggregation of Revenue [Line Items]
Total revenue	354	276	412
Austin, Texas
Disaggregation of Revenue [Line Items]
Total revenue	96,329	101,680	102,982
Austin, Texas | Rents
Disaggregation of Revenue [Line Items]
Total revenue	95,414	100,826	102,069
Austin, Texas | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	93,789	97,395	98,489
Austin, Texas | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	60,831	62,545	63,366
Austin, Texas | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	32,958	34,850	35,123
Austin, Texas | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	1,264	3,322	3,531
Austin, Texas | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	361	109	49
Austin, Texas | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	486	452	689
Austin, Texas | Other income
Disaggregation of Revenue [Line Items]
Total revenue	429	402	224
Metropolitan Washington, D.C.
Disaggregation of Revenue [Line Items]
Total revenue	21,899	19,865	40,223
Metropolitan Washington, D.C. | Rents
Disaggregation of Revenue [Line Items]
Total revenue	18,053	13,173	33,509
Metropolitan Washington, D.C. | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	17,969	13,056	33,374
Metropolitan Washington, D.C. | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	16,686	12,410	29,830
Metropolitan Washington, D.C. | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	1,283	646	3,544
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	84	117	135
Metropolitan Washington, D.C. | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	3,723	6,548	6,541
Metropolitan Washington, D.C. | Other income
Disaggregation of Revenue [Line Items]
Total revenue	123	144	173
Other
Disaggregation of Revenue [Line Items]
Total revenue	14,711	14,015	13,469
Other | Rents
Disaggregation of Revenue [Line Items]
Total revenue	12,685	10,913	10,900
Other | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	12,449	10,680	10,465
Other | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	9,805	8,020	8,064
Other | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	2,644	2,660	2,401
Other | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue	0	0	203
Other | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue	236	233	232
Other | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue	1,988	3,077	2,560
Other | Other income
Disaggregation of Revenue [Line Items]
Total revenue	38	25	9
Corporate
Disaggregation of Revenue [Line Items]
Total revenue	23,346	18,835	4,537
Corporate | Rents
Disaggregation of Revenue [Line Items]
Total revenue	(1,930)	(2,497)	(3,755)
Corporate | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue	(1,930)	(2,497)	(3,755)
Corporate | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue	(1,895)	(2,240)	(2,412)
Corporate | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue	(35)	(257)	(1,343)
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	17,613	15,440	7,824
Corporate | Other income
Disaggregation of Revenue [Line Items]
Total revenue	$ 7,663	$ 5,892	$ 468