For an unclassified balance sheet, amounts due from related parties including affiliates, employees, joint ventures, officers and stockholders, immediate families thereof, and pension funds.

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Investments in Unconsolidated Joint Ventures (Liberty/Comcast 1701 JFK Boulevard, LP) (Details) ft² in Thousands, $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Equity Method Investments [Line Items]
Repayments of Secured Debt		$ 7,795	$ 29,538	$ 175,364
Liberty Comcast 1701 JFK Boulevard, LP [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage		20.00%
Philadelphia [Member] \| Liberty Comcast 1701 JFK Boulevard, LP [Member]
Schedule of Equity Method Investments [Line Items]
Ownership percentage		20.00%
Philadelphia [Member] \| Equity Method Investee [Member] \| Liberty Comcast 1701 JFK Boulevard, LP [Member]
Schedule of Equity Method Investments [Line Items]
Management fees revenue		$ 2,700	2,700	$ 2,700
Philadelphia [Member] \| Building and Building Improvements [Member] \| Liberty Comcast 1701 JFK Boulevard, LP [Member]
Schedule of Equity Method Investments [Line Items]
Leaseable square feet \| ft²		1,250
Philadelphia [Member] \| Investments in and advances to unconsolidated joint ventures [Member] \| Equity Method Investee [Member] \| Liberty Comcast 1701 JFK Boulevard, LP [Member]
Schedule of Equity Method Investments [Line Items]
Due to related parties		$ 59	59
Philadelphia [Member] \| Prepaid Expenses and Other Current Assets [Member] \| Equity Method Investee [Member] \| Liberty Comcast 1701 JFK Boulevard, LP [Member]
Schedule of Equity Method Investments [Line Items]
Due from related parties		$ 272	$ 401
Subsequent Event [Member] \| Philadelphia [Member] \| Equity Method Investee [Member] \| Liberty Comcast 1701 JFK Boulevard, LP [Member]
Schedule of Equity Method Investments [Line Items]
Repayments of Secured Debt	$ 305,200