Unconsolidated Joint Ventures	6 Months Ended
Jun. 30, 2018
Equity Method Investments and Joint Ventures [Abstract]
Equity Method Investments and Joint Ventures Disclosure [Text Block]	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”) and a 217-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 $960 million. As of June 30, 2018, the Company's investment in these joint ventures was $188.2 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. During the six months ended June 30, 2018, the joint ventures for Liberty Property 18th & Arch brought into service 250,000 square feet of office space representing a Total Investment by the Office joint venture of $138.3 million. The office portion of the Project is now substantially complete. The 217-room Four Seasons hotel representing an aggregate Total Investment by the Hotel joint venture of $222.6 million when completed continued to be developed by the Hotel joint venture as of June 30, 2018. 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 (as that guaranteed maximum price has been adjusted pursuant to change orders accepted to date). The Company intends to vigorously 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, based upon information we have received from the third-party contractor since the Company filed its quarterly report on Form 10-Q for the period ended March 31, 2018, the Company has determined that it is probable that the Company may be required to initially fund cost overruns in compliance with its obligations under its development cost guarantee to the joint ventures. We currently estimate that these costs could total $60 million more than previously accrued and, as such, the Company has accrued such amount as additional development service fee expense in its consolidated statements of comprehensive income for the three months ended June 30, 2018. There can be no assurances that amounts incurred, including as a result of claims that have been or may be asserted by the third-party contractor, will not exceed these estimates. If the Company were to incur additional expenses in connection with its development cost guarantee, 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. Liberty/Comcast 1701 JFK Boulevard, LP During the six months ended June 30, 2018, Liberty/Comcast 1701 JFK Boulevard LP (a joint venture in which the Company holds a 20% interest) paid in full its $305.2 million mortgage loan. The payment was funded through loans from the joint venture partners in proportion to their ownership interests. The Company's portion of the loan to the joint venture is included in investments in, and advances to, unconsolidated joint ventures in the Company's consolidated balance sheets. Cambridge Medipark Ltd During the three and six months ended June 30, 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 $1.7 million and $3.7 million for the three and six months ended June 30, 2018, respectively, compared to $1.5 million and $4.5 million, respectively, for the same periods in 2017.

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”) and a 217-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 $960 million. As of June 30, 2018, the Company's investment in these joint ventures was $188.2 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.

During the six months ended June 30, 2018, the joint ventures for Liberty Property 18th & Arch brought into service 250,000 square feet of office space representing a Total Investment by the Office joint venture of $138.3 million. The office portion of the Project is now substantially complete. The 217-room Four Seasons hotel representing an aggregate Total Investment by the Hotel joint venture of $222.6 million when completed continued to be developed by the Hotel joint venture as of June 30, 2018.

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 (as that guaranteed maximum price has been adjusted pursuant to change orders accepted to date). The Company intends to vigorously 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, based upon information we have received from the third-party contractor since the Company filed its quarterly report on Form 10-Q for the period ended March 31, 2018, the Company has determined that it is probable that the Company may be required to initially fund cost overruns in compliance with its obligations under its development cost guarantee to the joint ventures. We currently estimate that these costs could total $60 million more than previously accrued and, as such, the Company has accrued such amount as additional development service fee expense in its consolidated statements of comprehensive income for the three months ended June 30, 2018.

There can be no assurances that amounts incurred, including as a result of claims that have been or may be asserted by the third-party contractor, will not exceed these estimates. If the Company were to incur additional expenses in connection with its development cost guarantee, 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.

Liberty/Comcast 1701 JFK Boulevard, LP

During the six months ended June 30, 2018, Liberty/Comcast 1701 JFK Boulevard LP (a joint venture in which the Company holds a 20% interest) paid in full its $305.2 million mortgage loan. The payment was funded through loans from the joint venture partners in proportion to their ownership interests. The Company's portion of the loan to the joint venture is included in investments in, and advances to, unconsolidated joint ventures in the Company's consolidated balance sheets.

Cambridge Medipark Ltd

During the three and six months ended June 30, 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 $1.7 million and $3.7 million for the three and six months ended June 30, 2018, respectively, compared to $1.5 million and $4.5 million, respectively, for the same periods in 2017.