Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Investment in real estate ventures	$ 367,771		$ 367,771		$ 120,294
Equity in loss of real estate ventures	(5,788)	$ (1,965)	(9,882)	$ (4,814)
Philadelphia CBD
Segment Reporting Information [Line Items]
Investment in real estate ventures	277,161		277,161		17,524
Equity in loss of real estate ventures	(2,737)	55	(2,575)	217
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Investment in real estate ventures	100,325		100,325		102,840
Equity in loss of real estate ventures	(1,367)	(528)	(2,463)	(963)
MAP Venture
Segment Reporting Information [Line Items]
Investment in real estate ventures	(9,715)		(9,715)		(70)
Equity in loss of real estate ventures	(1,684)	(1,602)	(4,844)	(4,414)
Other
Segment Reporting Information [Line Items]
Investment in real estate ventures	0		0		$ 0
Equity in loss of real estate ventures	$ 0	$ 110	$ 0	$ 346