Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands		Mar. 31, 2019		Jan. 01, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating properties		$ 4,015,879			$ 3,953,319
Right of use asset - operating leases		22,175	[1]	$ 22,400	0
Construction-in-progress		112,176			150,263
Land held for development	[2]	88,047			86,401
Prepaid leasehold interests in land held for development, net	[3]	39,897			39,999
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Operating properties		1,683,406			1,670,388
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Operating properties		1,008,066			1,004,537
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Operating properties		713,189			667,698
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Operating properties		524,385			524,190
Other [Member]
Segment Reporting Information [Line Items]
Operating properties		$ 86,833			$ 86,506

[1] On January 1, 2019, as a result of the adoption of Topic 842, Leases, the Company recognized operating ground leases for which it is a lessee on its consolidated balance sheets for the period ended March 31, 2019. See Note 2, “Basis of Presentation,” for further information.

[2] As of March 31, 2019, the Company categorized 35.2 acres of land held for development, located in the Other segment, as held for sale in accordance with applicable accounting standards for long lived assets. As of December 31, 2018, the Company categorized 37.9 acres of land held for development, comprised of 2.7 acres and 35.2 acres, located in the Pennsylvania Suburbs segment and Other segment, respectively, as held for sale in accordance with applicable accounting standards for long lived assets. See Note 3, “Real Estate Investments,” for further information.

[3] As of March 31, 2019 and December 31, 2018, this caption comprised leasehold interests in prepaid 99-year ground leases at 3025 and 3001-3003 JFK Boulevard, in Philadelphia, Pennsylvania.