Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Total revenue	$ 136,837	$ 144,151	$ 281,925	$ 288,047
Operating expenses	(52,482)	(57,222)	(109,392)	(114,622)
Net operating income (loss)	84,355	86,929	172,533	173,425
Operating Segments | Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	62,648	64,956	128,563	130,754
Operating expenses	(22,895)	(25,353)	(47,093)	(50,538)
Net operating income (loss)	39,753	39,603	81,470	80,216
Operating Segments | Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	34,683	34,299	71,920	69,926
Operating expenses	(11,161)	(11,808)	(23,839)	(24,799)
Net operating income (loss)	23,522	22,491	48,081	45,127
Operating Segments | Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	25,012	26,188	51,593	50,954
Operating expenses	(9,943)	(9,804)	(20,088)	(18,880)
Net operating income (loss)	15,069	16,384	31,505	32,074
Operating Segments | Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Total revenue	10,091	13,673	20,845	27,193
Operating expenses	(4,930)	(6,339)	(10,442)	(12,543)
Net operating income (loss)	5,161	7,334	10,403	14,650
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue	3,381	3,464	7,033	6,646
Operating expenses	(2,367)	(2,390)	(5,060)	(4,535)
Net operating income (loss)	1,014	1,074	1,973	2,111
Corporate
Segment Reporting Information [Line Items]
Total revenue	1,022	1,571	1,971	2,574
Operating expenses	(1,186)	(1,528)	(2,870)	(3,327)
Net operating income (loss)	$ (164)	$ 43	$ (899)	$ (753)