Basis of Presentation - Summary of Revenue Earned by Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Revenue Reconciling Item [Line Items]
Base rent	$ 95,343
Straight-line rent	5,205
Point of sale	5,812
Total rents	106,360	$ 103,332
Tenant reimbursements	19,849	18,535
Termination fees	550	1,673
Third party management fees, labor reimbursement and leasing	7,674	6,485
Other	1,925	895
Total revenue	136,358	130,920
Philadelphia CBD [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Base rent	39,175
Straight-line rent	4,339
Point of sale	5,341
Total rents	48,855
Tenant reimbursements	12,366
Termination fees	47
Third party management fees, labor reimbursement and leasing	212
Other	1,122
Total revenue	62,602
Pennsylvania Suburbs [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Base rent	29,894
Straight-line rent	499
Point of sale	48
Total rents	30,441
Tenant reimbursements	3,921
Termination fees	434
Third party management fees, labor reimbursement and leasing	8
Other	78
Total revenue	34,882
Metropolitan Washington, D.C. [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Base rent	20,079
Straight-line rent	208
Point of sale	240
Total rents	20,527
Tenant reimbursements	928
Termination fees	69
Third party management fees, labor reimbursement and leasing	1,465
Other	70
Total revenue	23,059
Austin, Texas [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Base rent	4,485
Straight-line rent	256
Point of sale	122
Total rents	4,863
Tenant reimbursements	1,981
Third party management fees, labor reimbursement and leasing	1,501
Other	19
Total revenue	8,364
Other [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Base rent	2,196
Straight-line rent	18
Point of sale	61
Total rents	2,275
Tenant reimbursements	765
Third party management fees, labor reimbursement and leasing	3,095
Other	6
Total revenue	6,141
Corporate [Member]
Segment Reporting Revenue Reconciling Item [Line Items]
Base rent	(486)
Straight-line rent	(115)
Total rents	(601)
Tenant reimbursements	(112)
Third party management fees, labor reimbursement and leasing	1,393
Other	630
Total revenue	$ 1,310	$ 1,969