Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating properties	$ 3,995,228		$ 3,951,719
Right of use asset - operating leases, net	21,828	$ 22,400	0
Construction-in-progress	151,232		150,263
Land held for development	92,189		86,401
Prepaid leasehold interests in land held for development, net	39,694		39,999
Philadelphia CBD
Segment Reporting Information [Line Items]
Operating properties	1,716,466		1,670,388
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Operating properties	1,005,332		1,002,937
Austin, Texas
Segment Reporting Information [Line Items]
Operating properties	719,441		667,698
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Operating properties	467,117		524,190
Other
Segment Reporting Information [Line Items]
Operating properties	$ 86,872		$ 86,506