Investment in Unconsolidated Real Estate Ventures - Summary of Results of Operations of Real Estate Ventures with Interests (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule Of Equity Method Investments [Line Items]
Equity in income (loss) of Real Estate Ventures	$ 1	$ (5,723)	$ (1,182)	$ (5,387)
DRA Austin Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue	14,232	22,493	42,492	70,185
Operating expenses	(6,428)	(10,107)	(18,245)	(29,119)
Interest expense, net	(2,549)	(3,784)	(7,070)	(11,217)
Depreciation and amortization	(4,896)	(9,334)	(15,622)	(27,627)
Loss on early extinguishment of debt	0		0
Net income (loss)	$ 359	$ (732)	$ 1,555	$ 2,222
Ownership interest %	50.00%	50.00%	50.00%	50.00%
Company's share of net income (loss)	$ 180	$ (366)	$ 778	$ 1,111
Other-than-temporary impairment		0		0
Basis adjustments and other	243	351	378	155
Equity in income (loss) of Real Estate Ventures	423	(15)	1,156	1,266
Brandywine - AI Venture LLC [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue	5,962	8,438	17,768	23,751
Operating expenses	(2,589)	(4,359)	(8,010)	(10,547)
Interest expense, net	(873)	(1,384)	(2,606)	(3,830)
Depreciation and amortization	(2,232)	(3,251)	(6,915)	(9,193)
Loss on early extinguishment of debt	0		0
Net income (loss)	$ 268	$ (556)	$ 237	$ 181
Ownership interest %	50.00%	50.00%	50.00%	50.00%
Company's share of net income (loss)	$ 134	$ (278)	$ 119	$ 91
Other-than-temporary impairment		(4,844)		(4,844)
Basis adjustments and other	31	(8)	33	291
Equity in income (loss) of Real Estate Ventures	165	(5,130)	152	(4,462)
HSRE-BDN I, LLC (evo at Cira Centre South) [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue	0	3,009	995	9,264
Operating expenses	0	(841)	(250)	(2,232)
Interest expense, net	0	(1,097)	(388)	(3,009)
Depreciation and amortization	0	(1,128)	(376)	(3,384)
Loss on early extinguishment of debt	0		(718)
Net income (loss)	$ 0	$ (57)	$ (737)	$ 639
Ownership interest %	50.00%	50.00%	50.00%	50.00%
Company's share of net income (loss)	$ 0	$ (29)	$ (369)	$ 320
Other-than-temporary impairment		0		0
Basis adjustments and other	0	53	11	105
Equity in income (loss) of Real Estate Ventures	0	24	(358)	425
Other Income [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue	21,823	22,571	64,684	66,409
Operating expenses	(12,123)	(12,111)	(35,492)	(35,858)
Interest expense, net	(4,122)	(5,406)	(13,558)	(15,716)
Depreciation and amortization	(6,135)	(6,970)	(18,526)	(21,612)
Loss on early extinguishment of debt	(334)		(334)
Net income (loss)	(891)	(1,916)	(3,226)	(6,777)
Company's share of net income (loss)	(550)	(1,002)	(1,946)	(2,909)
Other-than-temporary impairment		0		0
Basis adjustments and other	(37)	400	(186)	293
Equity in income (loss) of Real Estate Ventures	(587)	(602)	(2,132)	(2,616)
Unconsolidated Real Estate Ventures [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue	42,017	56,511	125,939	169,609
Operating expenses	(21,140)	(27,418)	(61,997)	(77,756)
Interest expense, net	(7,544)	(11,671)	(23,622)	(33,772)
Depreciation and amortization	(13,263)	(20,683)	(41,439)	(61,816)
Loss on early extinguishment of debt	(334)		(1,052)
Net income (loss)	(264)	(3,261)	(2,171)	(3,735)
Company's share of net income (loss)	(236)	(1,675)	(1,418)	(1,387)
Other-than-temporary impairment		(4,844)		(4,844)
Basis adjustments and other	237	796	236	844
Equity in income (loss) of Real Estate Ventures	$ 1	$ (5,723)	$ (1,182)	$ (5,387)