Unconsolidated Joint Ventures	3 Months Ended
Mar. 31, 2019
Equity Method Investments and Joint Ventures [Abstract]
Unconsolidated Joint Ventures	Unconsolidated Joint Ventures Liberty Property 18th & Arch LP and Liberty Property 18th & Arch Hotel, LP On June 30, 2014, the Company entered into two joint ventures for the purpose of developing and owning the Comcast Technology Center (the “Project”) located in Philadelphia, Pennsylvania as part of a mixed-use development. The 60-story building includes 1.3 million square feet of leasable office space (the “Office”), which is substantially complete, and will include a 219-room Four Seasons Hotel (the “Hotel”) (collectively, “Liberty Property 18th and Arch”). Project costs for the development of the Project, exclusive of tenant-funded interior improvements, are anticipated to be approximately $962 million. As of March 31, 2019, the Company's investment in the project is $193 million and is reflected in investments in and advances to unconsolidated joint ventures in the Company's consolidated balance sheet. These joint ventures are part of the Company's Philadelphia reportable segment. As of March 31, 2019 the Office was substantially complete and the Hotel, representing an aggregate Total Investment by the Hotel joint venture of $226.1 million when completed, continued to be developed by the Hotel joint venture. The two joint ventures have engaged the Company as the developer of the Project pursuant to a Development Agreement by which the Company agrees, in consideration for a development fee, to be responsible for all aspects of the development of the Project and to guarantee the timely lien-free completion of construction of the Project as well as the payment, subject to certain exceptions, of any cost overruns incurred in the development of the Project. To mitigate its risk, the Company entered into guaranteed maximum price contracts with a third party contractor (the "GMP Contracts") to construct the Project. The Company has been notified by its third-party contractor that the contractor has incurred cost overruns and expects to incur additional construction costs in connection with completing the Project in excess of the guaranteed maximum price payable to the contractor under the GMP Contracts, which guaranteed maximum price has been previously adjusted pursuant to accepted change orders. The Company intends to pursue all remedies to recover from the third-party contractor any amounts expended by the Company or the joint ventures in excess of their contractual obligations. However, the general contractor has generally refused to fund the additional costs, and the Company has funded, and may continue to fund, cost overruns in compliance with its obligations under its development cost guarantee to the joint ventures. Accordingly, reflective of amounts funded by the Company, as of March 31, 2019 and December 31, 2018, the Company's accrual was $44.5 million and $67.3 million, respectively, relating to the above-described development cost guarantees, which are included in other liabilities in the accompanying consolidated balance sheets. The Company is accounting for the development of the Project using the percentage of completion method. The Company recognized a loss of $1.8 million for the three months ended March 31, 2018 in development service fee income, net of development service fee expense and other related expenses. There was no loss or gain recognized related to the Project during the three months ended March 31, 2019. In addition to the costs to comply with the Company's obligations under its development cost guarantee, claims have been asserted by the third-party contractor and certain subcontractors. There can be no assurances that amounts incurred, including as a result of such claims, will not exceed the above estimates. The Company is not able to reasonably estimate the amount of additional expenses, if any, that it may incur as a result of such claims, and accordingly any potential exposure of the Company for such claims is not included within the accrual described above. If the Company were to incur additional expenses in connection with its development cost guarantee or in connection with such claims, such amounts would be accrued when they are determined to be probable of being incurred and are reasonably estimable, and could be material to the Company’s results of operations in future periods. If the Company were to subsequently recover any of the cost overruns initially funded by the Company, such recoveries would be recorded when and if realized in future periods. Cambridge Medipark Ltd During the three months ended March 31, 2019 and 2018, Cambridge Medipark, Ltd (a joint venture in which the Company holds a 50% interest) recognized gains on the sale of land leasehold interests. The Company's share of these gains was $775,000 and $2.0 million for the three months ended March 31, 2019 and 2018, respectively.

Unconsolidated Joint Ventures

Liberty Property 18th & Arch LP and Liberty Property 18th & Arch Hotel, LP

On June 30, 2014, the Company entered into two joint ventures for the purpose of developing and owning the Comcast Technology Center (the “Project”) located in Philadelphia, Pennsylvania as part of a mixed-use development. The 60-story building includes 1.3 million square feet of leasable office space (the “Office”), which is substantially complete, and will include a 219-room Four Seasons Hotel (the “Hotel”) (collectively, “Liberty Property 18th and Arch”). Project costs for the development of the Project, exclusive of tenant-funded interior improvements, are anticipated to be approximately $962 million. As of March 31, 2019, the Company's investment in the project is $193 million and is reflected in investments in and advances to unconsolidated joint ventures in the Company's consolidated balance sheet. These joint ventures are part of the Company's Philadelphia reportable segment.

As of March 31, 2019 the Office was substantially complete and the Hotel, representing an aggregate Total Investment by the Hotel joint venture of $226.1 million when completed, continued to be developed by the Hotel joint venture.

The two joint ventures have engaged the Company as the developer of the Project pursuant to a Development Agreement by which the Company agrees, in consideration for a development fee, to be responsible for all aspects of the development of the Project and to guarantee the timely lien-free completion of construction of the Project as well as the payment, subject to certain exceptions, of any cost overruns incurred in the development of the Project. To mitigate its risk, the Company entered into guaranteed maximum price contracts with a third party contractor (the "GMP Contracts") to construct the Project. The Company has been notified by its third-party contractor that the contractor has incurred cost overruns and expects to incur additional construction costs in connection with completing the Project in excess of the guaranteed maximum price payable to the contractor under the GMP Contracts, which guaranteed maximum price has been previously adjusted pursuant to accepted change orders. The Company intends to pursue all remedies to recover from the third-party contractor any amounts expended by the Company or the joint ventures in excess of their contractual obligations. However, the general contractor has generally refused to fund the additional costs, and the Company has funded, and may continue to fund, cost overruns in compliance with its obligations under its development cost guarantee to the joint ventures. Accordingly, reflective of amounts funded by the Company, as of March 31, 2019 and December 31, 2018, the Company's accrual was $44.5 million and $67.3 million, respectively, relating to the above-described development cost guarantees, which are included in other liabilities in the accompanying consolidated balance sheets. The Company is accounting for the development of the Project using the percentage of completion method. The Company recognized a loss of $1.8 million for the three months ended March 31, 2018 in development service fee income, net of development service fee expense and other related expenses. There was no loss or gain recognized related to the Project during the three months ended March 31, 2019.

In addition to the costs to comply with the Company's obligations under its development cost guarantee, claims have been asserted by the third-party contractor and certain subcontractors. There can be no assurances that amounts incurred, including as a result of such claims, will not exceed the above estimates. The Company is not able to reasonably estimate the amount of additional expenses, if any, that it may incur as a result of such claims, and accordingly any potential exposure of the Company for such claims is not included within the accrual described above. If the Company were to incur additional expenses in connection with its development cost guarantee or in connection with such claims, such amounts would be accrued when they are determined to be probable of being incurred and are reasonably estimable, and could be material to the Company’s results of operations in future periods. If the Company were to subsequently recover any of the cost overruns initially funded by the Company, such recoveries would be recorded when and if realized in future periods.

Cambridge Medipark Ltd

During the three months ended March 31, 2019 and 2018, Cambridge Medipark, Ltd (a joint venture in which the Company holds a 50% interest) recognized gains on the sale of land leasehold interests. The Company's share of these gains was $775,000 and $2.0 million for the three months ended March 31, 2019 and 2018, respectively.