Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating properties		$ 3,953,319	$ 3,832,348	$ 3,586,295
Assets held for sale	[1]	0	0	73,591
Operating Properties		3,953,319	3,832,348	3,659,886
Construction-in-progress		150,263	121,188	297,462
Land held for development	[2]	86,401	98,242	150,970
Prepaid leasehold interests in land held for development, net	[3]	39,999	0	0
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Operating properties		1,670,388	1,643,296	1,320,974
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Operating properties		1,004,537	958,796	1,005,446
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Operating properties	[4]	524,190	978,257	975,987
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Operating properties	[5]	667,698	163,653	146,794
Other [Member]
Segment Reporting Information [Line Items]
Operating properties		$ 86,506	$ 88,346	$ 137,094

[1] As of December 31, 2016, three office properties in the Metropolitan Washington, D.C. segment and two office properties in the Other segment were classified as held for sale in accordance with applicable accounting standards for long-lived assets. See the “2016” section of Note 3, “Real Estate Investments,” for further information.

[2] As of December 31, 2018, the Company classified 37.9 acres of land held for development, comprised of 2.7 acres and 35.2 acres, located in its 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. As of December 31, 2017, the Company classified 13.1 acres of land held for development located in its Other segment as held for sale in accordance with applicable accounting standards for long lived assets.

[3] As of December 31, 2018, this caption comprised leasehold interests in prepaid 99-year ground leases at 3025 and 3001-3003 JFK Boulevard, in Philadelphia, Pennsylvania. See Note 3, “Real Estate Investments,” for further information.

[4] On December 20, 2018, the Company contributed a portfolio of eight properties containing an aggregate of 1,293,197 square feet, located in its Metropolitan Washington, D.C. segment, known as the Rockpoint Portfolio, to the Herndon Innovation Center Metro Portfolio Venture, LLC for a gross sales price of $312.0 million. The Company and its partner own 15% and 85% interests in the Herndon Innovation Center Metro Portfolio Venture, LLC, respectively. The Herndon Innovation Center Metro Portfolio Venture, LLC funded the acquisition with $265.2 million of cash, which was distributed to the Company at closing. After funding its share of closing costs and working capital contributions of $2.2 million and $0.6 million, respectively, the Company received $262.4 million of cash proceeds at settlement. For further information related to this transaction, see the “Herndon Innovation Center Metro Portfolio Venture, LLC” section in Note 4, “Investment in Unconsolidated Real Estate Ventures,” for further information.

[5] On December 11, 2018, the Company acquired from DRA Advisors its 50% ownership interest in the G&I Austin Office LLC real estate venture for an aggregate purchase price of $535.1 million. The DRA Austin Venture owned twelve office properties containing an aggregate 1,570,123 square feet, located in Austin, Texas. As a result of the acquisition, the Company acquired complete ownership of the Austin Venture Portfolio. For further information related to this transaction, see the “Acquisitions” section in Note 3, “Real Estate Investments.”