Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Total revenue	$ 126,106	$ 145,331	$ 408,031	$ 433,378
Operating expenses	(48,999)	(56,074)	(158,391)	(170,696)
Net operating income (loss)	77,107	89,257	249,640	262,682
Operating Segments | Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	52,640	66,105	181,203	196,859
Operating expenses	(18,522)	(24,773)	(65,615)	(75,311)
Net operating income (loss)	34,118	41,332	115,588	121,548
Operating Segments | Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	34,649	34,818	106,569	104,744
Operating expenses	(11,141)	(11,667)	(34,980)	(36,466)
Net operating income (loss)	23,508	23,151	71,589	68,278
Operating Segments | Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	25,328	26,280	76,921	77,234
Operating expenses	(9,944)	(9,302)	(30,032)	(28,182)
Net operating income (loss)	15,384	16,978	46,889	49,052
Operating Segments | Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Total revenue	9,675	13,179	30,520	40,372
Operating expenses	(5,236)	(5,999)	(15,678)	(18,542)
Net operating income (loss)	4,439	7,180	14,842	21,830
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue	3,188	3,706	10,221	10,352
Operating expenses	(2,374)	(2,691)	(7,434)	(7,226)
Net operating income (loss)	814	1,015	2,787	3,126
Corporate
Segment Reporting Information [Line Items]
Total revenue	626	1,243	2,597	3,817
Operating expenses	(1,782)	(1,642)	(4,652)	(4,969)
Net operating income (loss)	$ (1,156)	$ (399)	$ (2,055)	$ (1,152)