Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended									12 Months Ended
		Dec. 31, 2018	[1]	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total revenue		$ 139,203		$ 134,998	$ 133,786	$ 136,358	$ 133,344	$ 128,438	$ 127,791	$ 130,920	$ 544,345		$ 520,493	$ 525,463
Operating expenses	[2]										(218,023)		(205,999)	(209,448)
Net operating income (loss)											326,322		314,494	316,015
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue											256,717
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Total revenue											138,279
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue											90,308
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue											38,665
Other [Member]
Segment Reporting Information [Line Items]
Total revenue											16,757
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue											256,717		226,673	200,245
Operating expenses	[2]										(99,449)		(88,818)	(78,708)
Net operating income (loss)											157,268		137,855	121,537
Operating Segments [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Total revenue											138,279		139,785	144,338
Operating expenses	[2]										(49,357)		(47,769)	(49,208)
Net operating income (loss)											88,922		92,016	95,130
Operating Segments [Member] | Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue											90,308		92,024	99,781
Operating expenses	[2]										(34,072)		(35,014)	(39,036)
Net operating income (loss)											56,236		57,010	60,745
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue											38,665		34,301	34,585
Operating expenses	[2]										(16,739)		(15,456)	(13,222)
Net operating income (loss)											21,926		18,845	21,363
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Total revenue											16,757		18,347	39,359
Operating expenses	[2]										(11,888)		(11,749)	(23,204)
Net operating income (loss)											4,869		6,598	16,155
Corporate [Member]
Segment Reporting Information [Line Items]
Total revenue											3,619	[3]	9,363	7,155
Operating expenses	[2]										(6,518)		(7,193)	(6,070)
Net operating income (loss)											$ (2,899)		$ 2,170	$ 1,085

[1] The increase in fourth quarter revenues primarily relates to the acquisition of the Austin Portfolio, located in Austin, Texas, on December 11, 2018. See Note 3, “Real Estate Investments,” for further information on this transaction.

[2] Includes property operating expense, real estate taxes and third party management expense.

[3] Corporate includes intercompany eliminations necessary to reconcile to consolidated Company totals.