Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Total revenue		$ 133,786	$ 127,791	$ 270,144	$ 258,711
Operating expenses	[1]	(52,547)	(50,618)	(109,051)	(101,699)
Net operating income (loss)		81,239	77,173	161,093	157,012
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue		63,524		126,126
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Total revenue		34,333		69,215
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue		23,199		46,258
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue		8,469		16,833
Other [Member]
Segment Reporting Information [Line Items]
Total revenue		3,339		9,480
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue		63,524	54,451	126,126	108,900
Operating expenses	[1]	(24,805)	(21,464)	(49,132)	(42,301)
Net operating income (loss)		38,719	32,987	76,994	66,599
Operating Segments [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Total revenue		34,333	35,157	69,215	70,812
Operating expenses	[1]	(12,117)	(11,240)	(25,081)	(23,824)
Net operating income (loss)		22,216	23,917	44,134	46,988
Operating Segments [Member] | Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue		23,199	22,749	46,258	46,111
Operating expenses	[1]	(8,392)	(8,464)	(17,151)	(17,847)
Net operating income (loss)		14,807	14,285	29,107	28,264
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue		8,469	8,763	16,833	17,886
Operating expenses	[1]	(3,395)	(4,132)	(6,918)	(7,705)
Net operating income (loss)		5,074	4,631	9,915	10,181
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Total revenue		3,339	4,160	9,480	10,522
Operating expenses	[1]	(2,200)	(2,580)	(7,165)	(6,436)
Net operating income (loss)		1,139	1,580	2,315	4,086
Corporate [Member]
Segment Reporting Information [Line Items]
Total revenue		922	2,511	2,232	4,480
Operating expenses	[1]	(1,638)	(2,738)	(3,604)	(3,586)
Net operating income (loss)		$ (716)	$ (227)	$ (1,372)	$ 894

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