Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Total revenue	$ 145,088	$ 143,896
Operating expenses	(56,910)	(57,400)
Net operating income (loss)	88,178	86,496
Operating Segments | Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	65,915	65,798
Operating expenses	(24,198)	(25,185)
Net operating income (loss)	41,717	40,613
Operating Segments | Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	37,237	35,627
Operating expenses	(12,678)	(12,991)
Net operating income (loss)	24,559	22,636
Operating Segments | Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	26,581	24,766
Operating expenses	(10,145)	(9,076)
Net operating income (loss)	16,436	15,690
Operating Segments | Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Total revenue	10,754	13,520
Operating expenses	(5,512)	(6,204)
Net operating income (loss)	5,242	7,316
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue	3,652	3,182
Operating expenses	(2,693)	(2,145)
Net operating income (loss)	959	1,037
Corporate
Segment Reporting Information [Line Items]
Total revenue	949	1,003
Operating expenses	(1,684)	(1,799)
Net operating income (loss)	$ (735)	$ (796)