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Net rentable area for properties owned.

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Unconsolidated Joint Ventures - Liberty Property 18th & Arch LP and Liberty Property 18th & Arch, Hotel, LP (Details) $ in Thousands, ft² in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) investment	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) investment	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2014 ft² room investment story
Schedule of Equity Method Investments [Line Items]
Investments in and advances to unconsolidated joint ventures	$ 347,880		$ 347,880		$ 350,981
Philadelphia \| Liberty Property 18th & Arch
Schedule of Equity Method Investments [Line Items]
Number of equity method investments (in investments) \| investment	2		2			2
Anticipated Aggregate Investment	$ 968,000		$ 968,000
Investments in and advances to unconsolidated joint ventures	193,700		193,700
Philadelphia \| Construction in Progress \| Liberty Property 18th & Arch
Schedule of Equity Method Investments [Line Items]
Number of stories in a high-rise building (in stories) \| story						60
Number of hotel rooms (in rooms) \| room						219
Leaseable square feet (in sqft) \| ft²						1.3
Other Liabilities \| Philadelphia \| Liberty Property 18th & Arch \| Guarantee Obligations
Schedule of Equity Method Investments [Line Items]
Loss contingency accrual	29,600	$ 69,300	29,600	$ 69,300	$ 67,300
Accounts Payable [Member] \| Philadelphia \| Liberty Property 18th & Arch \| Guarantee Obligations
Schedule of Equity Method Investments [Line Items]
Retention Payable	24,000		24,000
Equity Method Investee \| Liberty Property 18th & Arch \| Philadelphia
Schedule of Equity Method Investments [Line Items]
Net operating income	$ 0	$ 0	$ 0	$ (61,800)