Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Total revenue	$ 145,331	$ 134,998	$ 433,378	$ 405,142
Operating expenses	(56,074)	(52,897)	(170,696)	(161,986)
Net operating income (loss)	89,257	82,101	262,682	243,156
Operating Segments | Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	66,105	64,352	196,859	190,478
Operating expenses	(24,773)	(24,427)	(75,311)	(73,559)
Net operating income (loss)	41,332	39,925	121,548	116,919
Operating Segments | Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	34,818	34,745	104,744	103,960
Operating expenses	(11,667)	(11,956)	(36,466)	(37,075)
Net operating income (loss)	23,151	22,789	68,278	66,885
Operating Segments | Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	26,280	8,641	77,234	25,474
Operating expenses	(9,302)	(3,894)	(28,182)	(10,812)
Net operating income (loss)	16,978	4,747	49,052	14,662
Operating Segments | Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Total revenue	13,179	22,754	40,372	69,012
Operating expenses	(5,999)	(8,548)	(18,542)	(25,699)
Net operating income (loss)	7,180	14,206	21,830	43,313
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue	3,706	3,707	10,352	13,187
Operating expenses	(2,691)	(2,436)	(7,226)	(9,601)
Net operating income (loss)	1,015	1,271	3,126	3,586
Corporate
Segment Reporting Information [Line Items]
Total revenue	1,243	799	3,817	3,031
Operating expenses	(1,642)	(1,636)	(4,969)	(5,240)
Net operating income (loss)	$ (399)	$ (837)	$ (1,152)	$ (2,209)