Revenue Recognition Revenue is measured based on the allocation of the transaction price in a contract to satisfied performance obligations. The transaction price does not include any amounts collected on behalf of third parties. We recognize revenue when we satisfy a performance obligation by transferring control over a product or service to a customer. Payments for our products and services are generally due within normal commercial terms. The following is a description of principal activities from which we generate our revenue, separated by reportable segments. See Note 4 for a further discussion regarding our reportable segments. Railcar Leasing and Management Services Group In our Railcar Leasing and Management Services Group ("Leasing Group"), revenue from rentals and operating leases, including contracts that contain non-level fixed lease payments, is recognized monthly on a straight-line basis. Leases not classified as operating leases are generally considered sales-type leases as a result of an option to purchase. We review our operating lease receivables for collectibility on a regular basis, taking into consideration changes in factors such as the lessees payment history, the financial condition of the lessee, and business and economic conditions in the industry in which the lessee operates. In the event that the collectibility of a receivable with respect to any lessee is no longer probable, we de-recognize the revenue and related receivable and recognize future revenue only when the lessee makes a rental payment. Contingent rents are recognized when the contingency is resolved. Selling profit or loss associated with sales-type leases is recognized upon lease commencement, and a net investment in the sales-type lease is recorded on the Consolidated Balance Sheet. Interest income related to sales-type leases is recognized over the lease term using the effective interest method. Revenue is recognized from the sales of railcars from the lease fleet on a gross basis in leasing revenues and cost of revenues if the railcar has been owned for one year or less at the time of sale. Sales of railcars from the lease fleet owned for more than one year are recognized as a net gain or loss from the disposal of a long-term asset. Revenue from servicing and management agreements is recognized as each performance period occurs. We account for shipping and handling costs as activities to fulfill the promise to transfer the good; as such, these fees are recorded in revenue. The fees and costs of shipping and handling activities are accrued when the related performance obligation has been satisfied. Rail Products Group Our railcar manufacturing business recognizes revenue when the customer has submitted its certificate of acceptance and legal title of the railcar has passed to the customer. Certain long-term contracts for the sales of railcars include price adjustments based on steel-price indices; this amount represents variable consideration for which we are unable to estimate the final consideration until the railcar is delivered, at which time the pricing becomes fixed. Within our maintenance services business, revenue is recognized over time as repair and maintenance projects are completed, using an input approach based on the costs incurred relative to the total estimated costs of performing the project. We recorded contract assets of $6.2 million and $10.2 million as of September 30, 2019 and December 31, 2018 , respectively, related to unbilled revenues recognized on repair and maintenance services that have been performed, but for which the entire project has not yet been completed, and the railcar has not yet been shipped to the customer. These contract assets are included within the Receivables, net of allowance line in our Consolidated Balance Sheets. All Other Our highway products business recognizes revenue when the customer has accepted the product and legal title of the product has passed to the customer. Unsatisfied Performance Obligations The following table includes estimated revenue expected to be recognized in future periods related to performance obligations that are unsatisfied or partially satisfied as of September 30, 2019 and the percentage of the outstanding performance obligations as of September 30, 2019 expected to be delivered during the remainder of 2019 : Unsatisfied performance obligations at September 30, 2019 Total Amount Percent expected to be delivered in 2019 (in millions) Rail Products Group: Products: External Customers $ 1,628.3 Leasing Group 817.4 $ 2,445.7 33 % Maintenance Services $ 44.9 49 % Railcar Leasing and Management Services Group $ 90.0 5 % The remainder of the unsatisfied performance obligations for the Rail Products Group is expected to be delivered through 2023. Unsatisfied performance obligations for the Railcar Leasing and Management Services Group are related to servicing, maintenance, and management agreements and are expected to be performed through 2029.