Revenue Recognition Rental Revenue Rental revenue includes base rents that each tenant pays in accordance with the terms of its respective lease and is reported on a straight-line basis over the non-cancellable term of the lease which includes the effects of rent steps and rent abatements under the leases. In general, we commence rental revenue recognition when the tenant takes possession of the leased space or controls the physical use of the leased space and the leased space is substantially ready for its intended use. We account for all of our leases as operating leases. Deferred rent receivables, including free rental periods and leasing arrangements allowing for increased base rent payments, are accounted for in a manner that provides an even amount of fixed lease revenues over the respective non-cancellable lease terms. Differences between rental income recognized and amounts due under the respective lease agreements are recognized as an increase or decrease to deferred rent receivables. In addition to base rent, our tenants also generally will pay their pro rata share of increases in real estate taxes and operating expenses for the building over a base year. In some leases, in lieu of paying additional rent based upon increases in building operating expenses, the tenant will pay additional rent based upon increases in an index such as the Consumer Price Index over the index value in effect during a base year, or contain fixed percentage increases over the base rent to cover escalations. We will recognize rental revenue of acquired in-place above- and below-market leases at their fair values over the terms of the respective leases, including, for below-market leases, fixed option renewal periods, if any. Lease termination fees are recognized when the fees are determinable, tenant vacancy has occurred, collectability is reasonably assured, we have no continuing obligation to provide services to such former tenants and the payment is not subject to any conditions that must be met or waived. Observatory Revenue Revenues from the sale of Observatory tickets are recognized upon admission or ticket expirations. Deferred revenue related to unused and unexpired tickets as of December 31, 2018 and 2017 was $4.1 million and $4.1 million , respectively, and is included in deferred revenue and other liabilities on the consolidated balance sheets. Gains on Sale of Real Estate We record a gain on sale of real estate when title is conveyed to the buyer and we have no substantial economic involvement with the property. If the sales criteria for the full accrual method are not met, we defer some or all of the gain recognition and account for the continued operations of the property by applying the finance, leasing, profit sharing, deposit, installment or cost recovery methods, as appropriate, until the sales criteria are met. Third-Party Management and Other Fees We earn revenue arising from contractual agreements with related party entities for asset and property management services. This revenue is recognized as the related services are performed under the respective agreements in place.