Basis of Presentation - Summary of Revenue Earned by Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
		Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017
Segment Reporting Revenue Reconciling Item [Line Items]
Total revenue		$ 134,998		$ 128,438	$ 405,142		$ 387,149
Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Total revenue		64,352			190,478
Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Total revenue		34,745			103,960
Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Total revenue		22,754			69,012
Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Total revenue		8,641			25,474
Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Total revenue		3,707			13,187
Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Total revenue		799	[1]	2,408	3,031	[1]	6,888
Base Rent [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		98,591			291,762
Base Rent [Member] | Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		42,289			121,940
Base Rent [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		29,116			88,929
Base Rent [Member] | Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		20,635			61,575
Base Rent [Member] | Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		5,008			14,442
Base Rent [Member] | Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		2,035			6,340
Base Rent [Member] | Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue	[1]	(492)			(1,464)
Straight Line Rent [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		2,534			10,765
Straight Line Rent [Member] | Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		1,912			9,194
Straight Line Rent [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		1,202			2,208
Straight Line Rent [Member] | Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		(492)			(645)
Straight Line Rent [Member] | Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		(43)			164
Straight Line Rent [Member] | Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		65			189
Straight Line Rent [Member] | Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue	[1]	(110)			(345)
Point of Sale [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		6,455			19,070
Point of Sale [Member] | Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		5,905			17,526
Point of Sale [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		73			201
Point of Sale [Member] | Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		287			783
Point of Sale [Member] | Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		134			385
Point of Sale [Member] | Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		56			175
Point of Sale [Member] | Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue	[1]	0			0
Rents [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		107,580		102,557	321,597		307,446
Rents [Member] | Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		50,106			148,660
Rents [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		30,391			91,338
Rents [Member] | Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		20,430			61,713
Rents [Member] | Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		5,099			14,991
Rents [Member] | Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		2,156			6,704
Rents [Member] | Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue	[1]	(602)			(1,809)
Tenant Reimbursements [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue		20,557		17,239	59,094		53,812
Tenant Reimbursements [Member] | Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue		13,087			37,585
Tenant Reimbursements [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue		3,704			10,795
Tenant Reimbursements [Member] | Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue		902			2,687
Tenant Reimbursements [Member] | Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue		2,286			6,258
Tenant Reimbursements [Member] | Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue		698			2,124
Tenant Reimbursements [Member] | Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue	[1]	(120)			(355)
Termination Fees [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		498		$ 200	1,630		$ 2,013
Termination Fees [Member] | Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		37			192
Termination Fees [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		461			1,300
Termination Fees [Member] | Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		0			138
Termination Fees [Member] | Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		0			0
Termination Fees [Member] | Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue		0			0
Termination Fees [Member] | Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Lease revenue	[1]	0			0
Third Party Management Fees, Labor Reimbursement and Leasing [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue		4,944			17,531
Third Party Management Fees, Labor Reimbursement and Leasing [Member] | Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue		203			632
Third Party Management Fees, Labor Reimbursement and Leasing [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue		6			18
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		1,279			4,130
Third Party Management Fees, Labor Reimbursement and Leasing [Member] | Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue		1,223			4,155
Third Party Management Fees, Labor Reimbursement and Leasing [Member] | Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue		846			4,345
Third Party Management Fees, Labor Reimbursement and Leasing [Member] | Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue	[1]	1,387			4,251
Other Income [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue		1,419			5,290
Other Income [Member] | Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue		919			3,409
Other Income [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue		183			509
Other Income [Member] | Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue		143			344
Other Income [Member] | Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue		33			70
Other Income [Member] | Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue		7			14
Other Income [Member] | Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Service and other revenue	[1]	$ 134			$ 944

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