Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Total revenue	$ 120,418	$ 126,106	$ 361,289	$ 408,031
Operating expenses	(47,052)	(48,999)	(141,153)	(158,391)
Net operating income (loss)	73,366	77,107	220,136	249,640
Operating Segments | Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	51,587	52,640	154,259	181,203
Operating expenses	(18,823)	(18,522)	(54,493)	(65,615)
Net operating income (loss)	32,764	34,118	99,766	115,588
Operating Segments | Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	31,327	34,649	93,361	106,569
Operating expenses	(10,154)	(11,141)	(30,625)	(34,980)
Net operating income (loss)	21,173	23,508	62,736	71,589
Operating Segments | Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	24,813	25,328	76,501	76,921
Operating expenses	(9,580)	(9,944)	(29,262)	(30,032)
Net operating income (loss)	15,233	15,384	47,239	46,889
Operating Segments | Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Total revenue	5,224	9,675	14,711	30,520
Operating expenses	(3,806)	(5,236)	(11,907)	(15,678)
Net operating income (loss)	1,418	4,439	2,804	14,842
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue	3,638	3,188	10,513	10,221
Operating expenses	(2,534)	(2,374)	(7,344)	(7,434)
Net operating income (loss)	1,104	814	3,169	2,787
Corporate
Segment Reporting Information [Line Items]
Total revenue	3,829	626	11,944	2,597
Operating expenses	(2,155)	(1,782)	(7,522)	(4,652)
Net operating income (loss)	$ 1,674	$ (1,156)	$ 4,422	$ (2,055)