Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Total revenue		$ 144,151	$ 133,786	$ 288,047	$ 270,144
Operating expenses	[1]	(57,222)	(52,566)	(114,622)	(109,089)
Net operating income (loss)		86,929	81,220	173,425	161,055
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue		64,956	63,524	130,754	126,126
Operating expenses	[1]	(25,353)	(24,581)	(50,538)	(48,908)
Net operating income (loss)		39,603	38,943	80,216	77,218
Operating Segments [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Total revenue		34,299	34,333	69,926	69,215
Operating expenses	[1]	(11,808)	(11,997)	(24,799)	(24,980)
Net operating income (loss)		22,491	22,336	45,127	44,235
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue	[2]	26,188	8,469	50,954	16,833
Operating expenses	[1],[2]	(9,804)	(3,395)	(18,880)	(6,918)
Net operating income (loss)	[2]	16,384	5,074	32,074	9,915
Operating Segments [Member] | Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue	[3]	13,673	23,199	27,193	46,258
Operating expenses	[1],[3]	(6,339)	(8,392)	(12,543)	(17,151)
Net operating income (loss)	[3]	7,334	14,807	14,650	29,107
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Total revenue		3,464	3,339	6,646	9,480
Operating expenses	[1]	(2,390)	(2,200)	(4,535)	(7,165)
Net operating income (loss)		1,074	1,139	2,111	2,315
Corporate [Member]
Segment Reporting Information [Line Items]
Total revenue		1,571	922	2,574	2,232
Operating expenses	[1]	(1,528)	(2,001)	(3,327)	(3,967)
Net operating income (loss)		$ 43	$ (1,079)	$ (753)	$ (1,735)

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

[2] On December 11, 2018, the Company acquired from DRA Advisors, an unaffiliated third party, DRA’s 50% ownership interest in the G&I Austin Office LLC real estate venture. 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 properties.

[3] 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 Venture. The Company and its partner own 15% and 85% interests in the Herndon Innovation Center Venture, respectively.