Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Total revenue		$ 127,791	$ 127,181	$ 258,711	$ 263,683
Operating expenses	[1]	(50,618)	(50,221)	(101,699)	(104,996)
Net operating income (loss)		77,173	76,960	157,012	158,687
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue		54,451	48,082	108,900	97,752
Operating expenses	[1]	(21,464)	(19,775)	(42,301)	(39,031)
Net operating income (loss)		32,987	28,307	66,599	58,721
Operating Segments [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Total revenue		35,157	35,102	70,812	72,208
Operating expenses	[1]	(11,240)	(12,580)	(23,824)	(25,735)
Net operating income (loss)		23,917	22,522	46,988	46,473
Operating Segments [Member] | Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue		22,749	25,291	46,111	52,630
Operating expenses	[1]	(8,464)	(8,768)	(17,847)	(19,667)
Net operating income (loss)		14,285	16,523	28,264	32,963
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue		8,763	7,850	17,886	16,397
Operating expenses	[1]	(4,132)	(2,911)	(7,705)	(6,166)
Net operating income (loss)		4,631	4,939	10,181	10,231
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Total revenue		4,160	9,062	10,522	21,802
Operating expenses	[1]	(2,580)	(5,074)	(6,436)	(12,676)
Net operating income (loss)		1,580	3,988	4,086	9,126
Corporate [Member]
Segment Reporting Information [Line Items]
Total revenue		2,511	1,794	4,480	2,894
Operating expenses	[1]	(2,738)	(1,113)	(3,586)	(1,721)
Net operating income (loss)		$ (227)	$ 681	$ 894	$ 1,173

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