Investment in Unconsolidated Real Estate Ventures - Summary of Results of Operations of Real Estate Ventures with Interests (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule Of Equity Method Investments [Line Items]
Equity in income (loss) of Real Estate Ventures	$ (825)	$ (748)
DRA Austin Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue	14,356	21,552
Operating expenses	(6,066)	(9,313)
Interest expense, net	(2,294)	(3,689)
Depreciation and amortization	(5,698)	(9,229)
Net income (loss)	$ 298	$ (679)
Ownership interest %	50.00%	50.00%
Company's share of net income (loss)	$ 149	$ (340)
Basis adjustments and other	(69)	(25)
Equity in income (loss) of Real Estate Ventures	80	(365)
Brandywine - AI Venture LLC [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue	5,836	7,325
Operating expenses	(2,544)	(2,940)
Interest expense, net	(864)	(1,145)
Depreciation and amortization	(2,137)	(2,809)
Net income (loss)	$ 291	$ 431
Ownership interest %	50.00%	50.00%
Company's share of net income (loss)	$ 146	$ 216
Basis adjustments and other	15	310
Equity in income (loss) of Real Estate Ventures	161	526
HSRE-BDN I, LLC (evo at Cira Centre South) [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue	995	3,154
Operating expenses	(250)	(679)
Interest expense, net	(388)	(898)
Depreciation and amortization	(376)	(1,128)
Loss on early extinguishment of debt	(718)
Net income (loss)	$ (737)	$ 449
Ownership interest %	50.00%	50.00%
Company's share of net income (loss)	$ (369)	$ 225
Basis adjustments and other	11	7
Equity in income (loss) of Real Estate Ventures	(358)	232
Other [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue	21,634	22,248
Operating expenses	(11,908)	(12,234)
Interest expense, net	(4,580)	(5,098)
Depreciation and amortization	(6,173)	(7,595)
Net income (loss)	(1,027)	(2,679)
Company's share of net income (loss)	(632)	(1,262)
Basis adjustments and other	(76)	121
Equity in income (loss) of Real Estate Ventures	(708)	(1,141)
Unconsolidated Real Estate Ventures [Member]
Schedule Of Equity Method Investments [Line Items]
Revenue	42,821	54,279
Operating expenses	(20,768)	(25,166)
Interest expense, net	(8,126)	(10,830)
Depreciation and amortization	(14,384)	(20,761)
Loss on early extinguishment of debt	(718)
Net income (loss)	(1,175)	(2,478)
Company's share of net income (loss)	(706)	(1,161)
Basis adjustments and other	(119)	413
Equity in income (loss) of Real Estate Ventures	$ (825)	$ (748)