Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Total revenue		$ 130,920	$ 136,502
Operating expenses	[1]	(51,081)	(54,775)
Net operating income		79,839	81,727
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue		54,449	49,670
Operating expenses	[1]	(20,837)	(19,256)
Net operating income		33,612	30,414
Operating Segments [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Total revenue		35,655	37,106
Operating expenses	[1]	(12,584)	(13,155)
Net operating income		23,071	23,951
Operating Segments [Member] | Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue		23,362	27,339
Operating expenses	[1]	(9,383)	(10,899)
Net operating income		13,979	16,440
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue		9,123	8,547
Operating expenses	[1]	(3,573)	(3,255)
Net operating income		5,550	5,292
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Total revenue		6,362	12,740
Operating expenses	[1]	(3,856)	(7,602)
Net operating income		2,506	5,138
Corporate [Member]
Segment Reporting Information [Line Items]
Total revenue		1,969	1,100
Operating expenses	[1]	(848)	(608)
Net operating income		$ 1,121	$ 492

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