Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Investment in joint venture	$ 415,823		$ 389,811
Equity in loss of real estate ventures	(6,924)	$ (1,891)
Philadelphia CBD
Segment Reporting Information [Line Items]
Investment in joint venture	300,694		268,562
Equity in loss of real estate ventures	(4,279)	78
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Investment in joint venture	99,617		99,769
Equity in loss of real estate ventures	(417)	(449)
Mid-Atlantic Office Joint Venture
Segment Reporting Information [Line Items]
Investment in joint venture	32,735		32,996
Equity in loss of real estate ventures	207	0
MAP Venture
Segment Reporting Information [Line Items]
Investment in joint venture	(17,223)		$ (11,516)
Equity in loss of real estate ventures	$ (2,435)	$ (1,520)