Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Investment in real estate ventures	$ 127,759		$ 127,759		$ 169,100
Equity in (loss) income of Real Estate Ventures	(1,965)	$ 1	(4,814)	$ (1,182)
Philadelphia CBD
Segment Reporting Information [Line Items]
Investment in real estate ventures	18,114		18,114		19,897
Equity in (loss) income of Real Estate Ventures	55	(36)	217	(183)
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Investment in real estate ventures	106,111		106,111		136,142
Equity in (loss) income of Real Estate Ventures	(528)	(31)	(963)	(431)
MAP Venture
Segment Reporting Information [Line Items]
Investment in real estate ventures	1,620		1,620		11,173
Equity in (loss) income of Real Estate Ventures	(1,602)	(444)	(4,414)	(2,011)
Other
Segment Reporting Information [Line Items]
Investment in real estate ventures	1,914		1,914		1,888
Equity in (loss) income of Real Estate Ventures	110	89	346	287
Austin, Texas
Segment Reporting Information [Line Items]
Investment in real estate ventures	0		0		$ 0
Equity in (loss) income of Real Estate Ventures	$ 0	$ 423	$ 0	$ 1,156