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 ($)	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($)	Jun. 30, 2014
Schedule of Equity Method Investments [Line Items]
Repayments of secured debt		$ 33,561	$ 7,795	$ 29,538
Management fee income		$ 73,224	82,673	12,941
Liberty Comcast 1701 JFK Boulevard, LP
Schedule of Equity Method Investments [Line Items]
Ownership percentage		20.00%
Philadelphia \| Liberty Property 18th & Arch
Schedule of Equity Method Investments [Line Items]
Ownership percentage					20.00%
Philadelphia \| Liberty Comcast 1701 JFK Boulevard, LP
Schedule of Equity Method Investments [Line Items]
Ownership percentage		20.00%
Philadelphia \| Equity Method Investee \| Liberty Comcast 1701 JFK Boulevard, LP
Schedule of Equity Method Investments [Line Items]
Repayments of secured debt	$ 305,200
Due to related parties		$ 60,800
Management fee income		$ 2,800	$ 2,700	$ 2,700
Philadelphia \| Building and Building Improvements \| Liberty Property 18th & Arch
Schedule of Equity Method Investments [Line Items]
Leaseable square feet (in sqft) \| ft²		250	1,100
Philadelphia \| Building and Building Improvements \| Liberty Comcast 1701 JFK Boulevard, LP
Schedule of Equity Method Investments [Line Items]
Leaseable square feet (in sqft) \| ft²		1,260
Philadelphia \| Investments in and advances to unconsolidated joint ventures \| Equity Method Investee \| Liberty Comcast 1701 JFK Boulevard, LP
Schedule of Equity Method Investments [Line Items]
Due to related parties		$ 59	$ 59
Philadelphia \| Prepaid Expenses and Other Current Assets \| Equity Method Investee \| Liberty Comcast 1701 JFK Boulevard, LP
Schedule of Equity Method Investments [Line Items]
Due from related parties		$ 326	$ 272