Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Total revenue	$ 120,769	$ 145,088
Operating expenses	(46,674)	(56,910)
Net operating income (loss)	74,095	88,178
Operating Segments | Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	51,227	65,915
Operating expenses	(17,411)	(24,198)
Net operating income (loss)	33,816	41,717
Operating Segments | Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	31,740	37,237
Operating expenses	(10,694)	(12,678)
Net operating income (loss)	21,046	24,559
Operating Segments | Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	26,175	26,581
Operating expenses	(9,720)	(10,145)
Net operating income (loss)	16,455	16,436
Operating Segments | Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Total revenue	4,675	10,754
Operating expenses	(4,199)	(5,512)
Net operating income (loss)	476	5,242
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue	3,213	3,652
Operating expenses	(2,282)	(2,693)
Net operating income (loss)	931	959
Corporate
Segment Reporting Information [Line Items]
Total revenue	3,739	949
Operating expenses	(2,368)	(1,684)
Net operating income (loss)	$ 1,371	$ (735)