SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Total revenue	$ 147,039	$ 145,331	$ 144,151	$ 143,896	$ 139,203	$ 134,998	$ 133,786	$ 136,358	$ 580,417	$ 544,345	$ 520,493
Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue									411,524
Variable rent
Disaggregation of Revenue [Line Items]
Total revenue									114,224
Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue									525,748
Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue									8,857
Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue									20,060
Rents
Disaggregation of Revenue [Line Items]
Total revenue									554,665
Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue									19,626
Other income
Disaggregation of Revenue [Line Items]
Total revenue									6,126
Philadelphia CBD
Disaggregation of Revenue [Line Items]
Total revenue									263,769
Philadelphia CBD | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue									178,481
Philadelphia CBD | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue									58,580
Philadelphia CBD | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue									237,061
Philadelphia CBD | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue									3,745
Philadelphia CBD | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue									18,665
Philadelphia CBD | Rents
Disaggregation of Revenue [Line Items]
Total revenue									259,471
Philadelphia CBD | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue									876
Philadelphia CBD | Other income
Disaggregation of Revenue [Line Items]
Total revenue									3,422
Pennsylvania Suburbs
Disaggregation of Revenue [Line Items]
Total revenue									141,084
Pennsylvania Suburbs | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue									125,969
Pennsylvania Suburbs | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue									14,282
Pennsylvania Suburbs | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue									140,251
Pennsylvania Suburbs | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue									(12)
Pennsylvania Suburbs | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue									174
Pennsylvania Suburbs | Rents
Disaggregation of Revenue [Line Items]
Total revenue									140,413
Pennsylvania Suburbs | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue									43
Pennsylvania Suburbs | Other income
Disaggregation of Revenue [Line Items]
Total revenue									628
Austin, Texas
Disaggregation of Revenue [Line Items]
Total revenue									104,157
Austin, Texas | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue									62,232
Austin, Texas | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue									34,748
Austin, Texas | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue									96,980
Austin, Texas | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue									4,638
Austin, Texas | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue									165
Austin, Texas | Rents
Disaggregation of Revenue [Line Items]
Total revenue									101,783
Austin, Texas | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue									1,956
Austin, Texas | Other income
Disaggregation of Revenue [Line Items]
Total revenue									418
Metropolitan DC
Disaggregation of Revenue [Line Items]
Total revenue									51,498
Metropolitan DC | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue									39,420
Metropolitan DC | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue									4,029
Metropolitan DC | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue									43,449
Metropolitan DC | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue									0
Metropolitan DC | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue									824
Metropolitan DC | Rents
Disaggregation of Revenue [Line Items]
Total revenue									44,273
Metropolitan DC | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue									6,922
Metropolitan DC | Other income
Disaggregation of Revenue [Line Items]
Total revenue									303
Other
Disaggregation of Revenue [Line Items]
Total revenue									14,558
Other | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue									7,834
Other | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue									3,080
Other | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue									10,914
Other | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue									486
Other | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue									232
Other | Rents
Disaggregation of Revenue [Line Items]
Total revenue									11,632
Other | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue									2,915
Other | Other income
Disaggregation of Revenue [Line Items]
Total revenue									11
Corporate
Disaggregation of Revenue [Line Items]
Total revenue									5,351
Corporate | Fixed rent
Disaggregation of Revenue [Line Items]
Total revenue									(2,412)
Corporate | Variable rent
Disaggregation of Revenue [Line Items]
Total revenue									(495)
Corporate | Total lease revenue
Disaggregation of Revenue [Line Items]
Total revenue									(2,907)
Corporate | Amortization of deferred market rents
Disaggregation of Revenue [Line Items]
Total revenue									0
Corporate | Daily parking & hotel flexible stay
Disaggregation of Revenue [Line Items]
Total revenue									0
Corporate | Rents
Disaggregation of Revenue [Line Items]
Total revenue									(2,907)
Corporate | Third party management fees, labor reimbursement and leasing
Disaggregation of Revenue [Line Items]
Total revenue									6,914
Corporate | Other income
Disaggregation of Revenue [Line Items]
Total revenue									$ 1,344