Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Total revenue		$ 136,358	$ 130,920
Operating expenses	[1]	(56,504)	(51,081)
Net operating income (loss)		79,854	79,839
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue		62,602
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Total revenue		34,882
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue		23,059
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue		8,364
Other [Member]
Segment Reporting Information [Line Items]
Total revenue		6,141
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue		62,602	54,449
Operating expenses	[1]	(24,327)	(20,837)
Net operating income (loss)		38,275	33,612
Operating Segments [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Total revenue		34,882	35,655
Operating expenses	[1]	(12,964)	(12,584)
Net operating income (loss)		21,918	23,071
Operating Segments [Member] | Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue		23,059	23,362
Operating expenses	[1]	(8,759)	(9,383)
Net operating income (loss)		14,300	13,979
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue		8,364	9,123
Operating expenses	[1]	(3,523)	(3,573)
Net operating income (loss)		4,841	5,550
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Total revenue		6,141	6,362
Operating expenses	[1]	(4,965)	(3,856)
Net operating income (loss)		1,176	2,506
Corporate [Member]
Segment Reporting Information [Line Items]
Total revenue		1,310	1,969
Operating expenses	[1]	(1,966)	(848)
Net operating income (loss)		$ (656)	$ 1,121

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