Subsequent Events (Textual) (Details) $ in Thousands			12 Months Ended
		Jan. 10, 2018 USD ($) a property	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jan. 05, 2018 USD ($) ft²
Subsequent Event [Line Items]
Cash proceeds after the payment of share of the debt and closing costs			$ 145,416	$ 21,022	$ 6,100
Four Tower Bridge [Member]
Subsequent Event [Line Items]
Ownership percentage	[1],[2]		65.00%
Seven Tower Bridge [Member]
Subsequent Event [Line Items]
Ownership percentage	[1],[2]		20.00%
Subsequent Event [Member] | Evo at Cira Disposition [Member]
Subsequent Event [Line Items]
Sale of ownership interest percentage		50.00%
Gross sales price		$ 197,500
Number of property units sold | property		345
Cash proceeds after the payment of share of the debt and closing costs		$ 43,000
Subsequent Event [Member] | Philadelphia [Member] | Pennsylvania [Member] | Evo at Cira Disposition [Member]
Subsequent Event [Line Items]
Principal balance of secured loan		110,900
Subsequent Event [Member] | Durham, North Carolina [Member] | Westpark Land [Member]
Subsequent Event [Line Items]
Gross sales price		$ 500
Acreage of land | a		13.1
Subsequent Event [Member] | Four Tower Bridge [Member]
Subsequent Event [Line Items]
Ownership percentage						35.00%
Subsequent Event [Member] | Four Tower Bridge [Member] | Conshohocken [Member] | Pennsylvania [Member]
Subsequent Event [Line Items]
Area of Real Estate Property | ft²						86,021
Real estate property debt						$ 9,700
Subsequent Event [Member] | Seven Tower Bridge [Member]
Subsequent Event [Line Items]
Sale of ownership interest percentage						20.00%

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

[2] Subsequent to December 31, 2017, the Company exchanged its 20% interest in Seven Tower Bridge to acquire the remaining 35% interest in Four Tower Bridge. See Note 20, “Subsequent Events,” to the consolidated financial statements for further information.