Segment Information - Unconsolidated Real Estate Ventures 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	Dec. 31, 2020
Segment Reporting Information [Line Items]
Investment in real estate ventures	$ 427,411		$ 427,411		$ 389,811
Equity in income (loss) of real estate venture	(6,634)	$ (5,788)	(20,798)	$ (9,882)
Philadelphia CBD
Segment Reporting Information [Line Items]
Investment in real estate ventures	319,645		319,645		268,562
Equity in income (loss) of real estate venture	(3,602)	(2,737)	(12,542)	(2,576)
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Investment in real estate ventures	97,972		97,972		99,769
Equity in income (loss) of real estate venture	(1,173)	(1,367)	(2,368)	(2,463)
Mid-Atlantic Office Joint Venture
Segment Reporting Information [Line Items]
Investment in real estate ventures	32,041		32,041		32,996
Equity in income (loss) of real estate venture	310	0	634	0
MAP Venture
Segment Reporting Information [Line Items]
Investment in real estate ventures	(22,247)		(22,247)		$ (11,516)
Equity in income (loss) of real estate venture	$ (2,169)	$ (1,684)	$ (6,522)	$ (4,843)