Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Investment in real estate ventures	$ 118,408		$ 120,294
Equity in loss of real estate ventures	(1,891)	$ (1,358)
Philadelphia CBD
Segment Reporting Information [Line Items]
Investment in real estate ventures	17,602		17,524
Equity in loss of real estate ventures	78	78
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Investment in real estate ventures	102,396		102,840
Equity in loss of real estate ventures	(449)	(175)
MAP Venture
Segment Reporting Information [Line Items]
Investment in real estate ventures	(1,590)		(70)
Equity in loss of real estate ventures	(1,520)	(1,367)
Other
Segment Reporting Information [Line Items]
Investment in real estate ventures	0		$ 0
Equity in loss of real estate ventures	$ 0	$ 106