Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Total revenue	$ 120,102	$ 136,837	$ 240,871	$ 281,925
Operating expenses	(47,427)	(52,482)	(94,101)	(109,392)
Net operating income (loss)	72,675	84,355	146,770	172,533
Operating Segments | Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	51,445	62,648	102,672	128,563
Operating expenses	(18,259)	(22,895)	(35,670)	(47,093)
Net operating income (loss)	33,186	39,753	67,002	81,470
Operating Segments | Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	30,294	34,683	62,034	71,920
Operating expenses	(9,777)	(11,161)	(20,471)	(23,839)
Net operating income (loss)	20,517	23,522	41,563	48,081
Operating Segments | Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	25,513	25,012	51,688	51,593
Operating expenses	(9,962)	(9,943)	(19,682)	(20,088)
Net operating income (loss)	15,551	15,069	32,006	31,505
Operating Segments | Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Total revenue	4,812	10,091	9,487	20,845
Operating expenses	(3,902)	(4,930)	(8,101)	(10,442)
Net operating income (loss)	910	5,161	1,386	10,403
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue	3,662	3,381	6,875	7,033
Operating expenses	(2,528)	(2,367)	(4,810)	(5,060)
Net operating income (loss)	1,134	1,014	2,065	1,973
Corporate
Segment Reporting Information [Line Items]
Total revenue	4,376	1,022	8,115	1,971
Operating expenses	(2,999)	(1,186)	(5,367)	(2,870)
Net operating income (loss)	$ 1,377	$ (164)	$ 2,748	$ (899)