Segment Information - Net Operating Income of Company's 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 Information [Line Items]
Total revenue		$ 134,998		$ 128,438	$ 405,142		$ 387,149
Operating expenses	[1]	(52,878)		(50,701)	(161,929)		(152,400)
Net operating income (loss)		82,120		77,737	243,213		234,749
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue		64,352			190,478
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Total revenue		34,745			103,960
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue		22,754			69,012
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue		8,641			25,474
Other [Member]
Segment Reporting Information [Line Items]
Total revenue		3,707			13,187
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue		64,352		56,452	190,478		165,352
Operating expenses	[1]	(24,427)		(22,010)	(73,559)		(64,311)
Net operating income (loss)		39,925		34,442	116,919		101,041
Operating Segments [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Total revenue		34,745		34,861	103,960		105,673
Operating expenses	[1]	(11,937)		(11,846)	(37,018)		(35,670)
Net operating income (loss)		22,808		23,015	66,942		70,003
Operating Segments [Member] | Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue		22,754		23,079	69,012		69,190
Operating expenses	[1]	(8,548)		(8,500)	(25,699)		(26,347)
Net operating income (loss)		14,206		14,579	43,313		42,843
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue		8,641		7,886	25,474		25,772
Operating expenses	[1]	(3,894)		(3,929)	(10,812)		(11,634)
Net operating income (loss)		4,747		3,957	14,662		14,138
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Total revenue		3,707		3,752	13,187		14,274
Operating expenses	[1]	(2,436)		(2,647)	(9,601)		(9,083)
Net operating income (loss)		1,271		1,105	3,586		5,191
Corporate [Member]
Segment Reporting Information [Line Items]
Total revenue		799	[2]	2,408	3,031	[2]	6,888
Operating expenses	[1]	(1,636)		(1,769)	(5,240)		(5,355)
Net operating income (loss)		$ (837)		$ 639	$ (2,209)		$ 1,533

[1] Includes property operating expenses, real estate taxes and third party management expense.

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