Segment Information - Unconsolidated Real Estate Ventures 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	Dec. 31, 2019
Segment Reporting Information [Line Items]
Investment in real estate ventures	$ 113,055		$ 113,055		$ 120,294
Equity in loss of real estate ventures	(2,203)	$ (1,491)	(4,094)	$ (2,849)
Philadelphia CBD
Segment Reporting Information [Line Items]
Investment in real estate ventures	16,611		16,611		17,524
Equity in loss of real estate ventures	83	84	161	162
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Investment in real estate ventures	101,947		101,947		102,840
Equity in loss of real estate ventures	(647)	(260)	(1,096)	(435)
MAP Venture
Segment Reporting Information [Line Items]
Investment in real estate ventures	(5,503)		(5,503)		(70)
Equity in loss of real estate ventures	(1,639)	(1,445)	(3,159)	(2,812)
Other
Segment Reporting Information [Line Items]
Investment in real estate ventures	0		0		$ 0
Equity in loss of real estate ventures	$ 0	$ 130	$ 0	$ 236