Investment in Unconsolidated Real Estate Ventures - Summary of Results of Operations of Real Estate Ventures with Interests (Details) - USD ($) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016		Mar. 02, 2016	Jan. 25, 2013
Schedule Of Equity Method Investments [Line Items]
Equity in income (loss) of Real Estate Ventures			$ (15,231)		$ (8,306)		$ (11,503)
DRA Austin Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue			53,476		85,500		85,749
Operating expenses			(22,994)		(35,997)		(37,643)
Interest expense, net			(9,083)		(13,985)		(15,052)
Depreciation and amortization			(19,226)		(34,026)		(38,365)
Provision for impairment			0				0	[1]
Loss on extinguishment of debt			(356)		(2,613)
Net income (loss)			$ 1,817		$ (1,121)		$ (5,311)	[2]
Ownership interest %			50.00%		50.00%		50.00%
Company's share of net income (loss)			$ 909		$ (560)		$ (2,656)
Other-than-temporary impairment	[3]		0		0
Basis adjustments and other			778		(429)		776
Equity in income (loss) of Real Estate Ventures			1,687		(989)		(1,880)
Brandywine - AI Venture LLC [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue			23,515		29,500		31,047
Operating expenses			(10,483)		(12,298)		(13,654)
Interest expense, net			(3,478)		(4,707)		(5,825)
Depreciation and amortization			(8,991)		(11,428)		(12,811)
Provision for impairment			(20,832)				(10,476)	[1]
Loss on extinguishment of debt			(695)		(811)
Net income (loss)			$ (20,964)		$ 256		$ (11,719)	[2]
Ownership interest %			50.00%	[4],[5]	50.00%		50.00%
Company's share of net income (loss)			$ (10,482)		$ 128		$ (5,860)
Other-than-temporary impairment		$ (4,800)	(4,076)	[3]	(4,844)	[3]
Basis adjustments and other			(1)		251		(35)
Equity in income (loss) of Real Estate Ventures			(14,559)	[5]	(4,465)	[5]	(5,895)
evo at Cira Centre South Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue			163		12,285		12,393
Operating expenses			(256)		(3,075)		(3,183)
Interest expense, net			(123)		(4,092)		(3,230)
Depreciation and amortization			(409)		(4,512)		(4,512)
Provision for impairment			0				0	[1]
Loss on extinguishment of debt			0		0
Net income (loss)			$ (625)		$ 606		$ 1,468	[2]
Ownership interest %			50.00%		50.00%		50.00%		50.00%	30.00%
Company's share of net income (loss)			$ (313)		$ 303		$ 734
Other-than-temporary impairment	[3]		0		0
Basis adjustments and other			(45)		146		109
Equity in income (loss) of Real Estate Ventures			(358)		449		843
Other Income [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue			88,172		88,986		85,263
Operating expenses			(48,302)		(47,970)		(48,712)
Interest expense, net			(17,090)		(17,429)		(16,207)
Depreciation and amortization			(25,200)		(28,474)		(30,050)
Provision for impairment			0				0	[1]
Loss on extinguishment of debt			(334)		0
Net income (loss)			(2,754)		(4,887)		(9,706)	[2]
Company's share of net income (loss)			(2,038)		(1,735)		(4,719)
Other-than-temporary impairment	[3]		0		0
Basis adjustments and other			37		(1,566)		148
Equity in income (loss) of Real Estate Ventures			(2,001)		(3,301)		(4,571)
Unconsolidated Real Estate Ventures [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue			165,326		216,271		214,452
Operating expenses			(82,035)		(99,340)		(103,192)
Interest expense, net			(29,774)		(40,213)		(40,314)
Depreciation and amortization			(53,826)		(78,440)		(85,738)
Provision for impairment			(20,832)				(10,476)	[1]
Loss on extinguishment of debt			(1,385)		(3,424)
Net income (loss)			(22,526)		(5,146)		(25,268)	[2]
Company's share of net income (loss)			(11,924)		(1,864)		(12,501)
Other-than-temporary impairment	[3]		(4,076)		(4,844)
Basis adjustments and other			769		(1,598)		998
Equity in income (loss) of Real Estate Ventures			$ (15,231)		$ (8,306)		$ (11,503)

[1] See “Brandywine - AI Venture: Station Square Impairment” section below.

[2] During the year ended December 31, 2016, there were $7.1 million of acquisition deal costs related to the formation of the MAP Venture.

[3] See “Brandywine - AI Venture: Other Than Temporary Impairment” section below.

[4] Ownership percentage represents the Company’s entitlement to residual distributions after payments of priority returns, where applicable.

[5] See “Brandywine - AI Venture: Station Square and 7101 Wisconsin Avenue” and “Brandywine - AI Venture: Other Than Temporary Impairment” sections below for information discussing activity that occurred during 2018 and 2017 relating to this venture.