Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Total revenue		$ 128,438	$ 129,694	$ 387,149	$ 393,377
Operating expenses	[1]	(50,701)	(51,317)	(152,400)	(156,313)
Net operating income (loss)		77,737	78,377	234,749	237,064
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue		56,452	50,744	165,352	148,496
Operating expenses	[1]	(22,010)	(19,071)	(64,311)	(58,102)
Net operating income (loss)		34,442	31,673	101,041	90,394
Operating Segments [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Total revenue		34,861	35,763	105,673	107,971
Operating expenses	[1]	(11,846)	(11,247)	(35,670)	(36,982)
Net operating income (loss)		23,015	24,516	70,003	70,989
Operating Segments [Member] | Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue		23,079	24,251	69,190	76,881
Operating expenses	[1]	(8,500)	(9,900)	(26,347)	(29,567)
Net operating income (loss)		14,579	14,351	42,843	47,314
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue		7,886	8,726	25,772	25,123
Operating expenses	[1]	(3,929)	(3,523)	(11,634)	(9,689)
Net operating income (loss)		3,957	5,203	14,138	15,434
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Total revenue		3,752	8,526	14,274	30,328
Operating expenses	[1]	(2,647)	(4,837)	(9,083)	(17,513)
Net operating income (loss)		1,105	3,689	5,191	12,815
Corporate [Member]
Segment Reporting Information [Line Items]
Total revenue		2,408	1,684	6,888	4,578
Operating expenses	[1]	(1,769)	(2,739)	(5,355)	(4,460)
Net operating income (loss)		$ 639	$ (1,055)	$ 1,533	$ 118

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