Summary of Significant Accounting Policies - Summary of Revenue Earned by Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended									12 Months Ended
		Dec. 31, 2018	[1]	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Revenue Reconciling Item [Line Items]
Total revenue		$ 139,203		$ 134,998	$ 133,786	$ 136,358	$ 133,344	$ 128,438	$ 127,791	$ 130,920	$ 544,345		$ 520,493	$ 525,463
Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Total revenue											256,717
Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Total revenue											138,279
Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Total revenue											90,308
Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Total revenue											38,665
Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Total revenue											16,757
Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Total revenue											3,619	[2]	9,363	7,155
Base Rent [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											392,048
Base Rent [Member] | Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											164,369
Base Rent [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											118,628
Base Rent [Member] | Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											80,858
Base Rent [Member] | Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											21,757
Base Rent [Member] | Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											8,400
Base Rent [Member] | Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue	[2]										(1,964)
Straight Line Rent [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											13,009
Straight Line Rent [Member] | Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											11,867
Straight Line Rent [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											2,851
Straight Line Rent [Member] | Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											(1,577)
Straight Line Rent [Member] | Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											116
Straight Line Rent [Member] | Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											201
Straight Line Rent [Member] | Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue	[2]										(449)
Point of Sale [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											25,599
Point of Sale [Member] | Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											23,528
Point of Sale [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											258
Point of Sale [Member] | Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											1,062
Point of Sale [Member] | Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											521
Point of Sale [Member] | Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											230
Point of Sale [Member] | Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue	[2]										0
Rents [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											430,656		412,333	421,505
Rents [Member] | Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											199,764
Rents [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											121,737
Rents [Member] | Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											80,343
Rents [Member] | Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											22,394
Rents [Member] | Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											8,831
Rents [Member] | Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue	[2]										(2,413)
Tenant Reimbursements [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											82,625		72,620	70,629
Tenant Reimbursements [Member] | Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											51,572
Tenant Reimbursements [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											14,440
Tenant Reimbursements [Member] | Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											3,519
Tenant Reimbursements [Member] | Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											10,736
Tenant Reimbursements [Member] | Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											2,832
Tenant Reimbursements [Member] | Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue	[2]										(474)
Termination Fees [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											1,763		$ 2,370	$ 2,339
Termination Fees [Member] | Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											192
Termination Fees [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											1,435
Termination Fees [Member] | Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											136
Termination Fees [Member] | Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											0
Termination Fees [Member] | Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue											0
Termination Fees [Member] | Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue	[2]										0
Third Party Management Fees, Labor Reimbursement and Leasing [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											22,557
Third Party Management Fees, Labor Reimbursement and Leasing [Member] | Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											855
Third Party Management Fees, Labor Reimbursement and Leasing [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											25
Third Party Management Fees, Labor Reimbursement and Leasing [Member] | Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											5,793
Third Party Management Fees, Labor Reimbursement and Leasing [Member] | Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											5,455
Third Party Management Fees, Labor Reimbursement and Leasing [Member] | Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											4,951
Third Party Management Fees, Labor Reimbursement and Leasing [Member] | Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue	[2]										5,478
Other Income [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											6,744
Other Income [Member] | Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											4,334
Other Income [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											642
Other Income [Member] | Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											517
Other Income [Member] | Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											80
Other Income [Member] | Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue											143
Other Income [Member] | Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue	[2]										$ 1,028

[1] The increase in fourth quarter revenues primarily relates to the acquisition of the Austin Portfolio, located in Austin, Texas, on December 11, 2018. See Note 3, “Real Estate Investments,” for further information on this transaction.

[2] Corporate includes intercompany eliminations necessary to reconcile to consolidated Company totals.