E. I. du Pont de Nemours and Company Notes to the Consolidated Financial Statements (continued) The company assesses the payment/performance risk by assigning default rates based on the duration of the guarantees. These default rates are assigned based on the external credit rating of the counterparty or through internal credit analysis and historical default history for counterparties that do not have published credit ratings. For counterparties without an external rating or available credit history, a cumulative average default rate is used. In certain cases, the company has recourse to assets held as collateral, as well as personal guarantees from customers and suppliers. Assuming liquidation, these assets are estimated to cover approximately 17 percent of the $90 million of guaranteed obligations of customers and suppliers. Set forth below are the company's guaranteed obligations at December 31, 2018. The following tables provide a summary of the final expiration and maximum future payments for each type of guarantee: 1. Existing guarantees for select customers, as part of contractual agreements. The terms of the guarantees are equivalent to the terms of the customer loans that are primarily made to finance customer invoices. Of the total maximum future payments, $89 million had terms less than a year. 2. Existing guarantees for non-consolidated affiliates' liquidity needs in normal operations. 3. The company provides guarantees related to leased assets specifying the residual value that will be available to the lessor at lease termination through sale of the assets to the lessee or third parties. The company uses various leased facilities and equipment in its operations. The terms for these leased assets vary depending on the lease agreement. Future minimum lease payments under non-cancelable operating leases are $242 million, $128 million, $90 million, $66 million and $44 million for the years 2019, 2020, 2021, 2022 and 2023, respectively, and $85 million for subsequent years and are not reduced by non-cancelable minimum sublease rentals due in the future in the amount of $2 million. Net rental expense under operating leases was $271 million, $105 million, $179 million and $242 million for the year ended December 31, 2018, for periods September 1 through December 31, 2017 and January 1 through August 31, 2017, and the year ended December 31, 2016, respectively. The company is subject to various legal proceedings arising out of the normal course of its current and former business operations, including product liability, intellectual property, commercial, environmental and antitrust lawsuits. It is not possible to predict the outcome of these various proceedings. Although considerable uncertainty exists, management does not anticipate that the ultimate disposition of these matters will have a material adverse effect on the company's results of operations, consolidated financial position or liquidity. However, the ultimate liabilities could be material to results of operations in the period recognized. PFOA Liabilities Historical DuPont is a party to legal proceedings relating to the use of PFOA (collectively, perfluorooctanoic acids and its salts, including the ammonium salt) by its former Performance Chemicals segment, which separated from DuPont in July 2015 through the spin-off of all the issued and outstanding stock of Chemours. While it is reasonably possible that the company could incur liabilities related to PFOA, any such liabilities are not expected to be material. As discussed in Note 4 and below, the company is indemnified by Chemours under the Chemours Separation Agreement, as amended. The company has recorded a liability of $20 million and an indemnification asset of $20 million at December 31, 2018, primarily related to testing drinking water in and around certain historic company sites and offering treatment or an alternative supply of drinking water if tests indicate the presence of PFOA in drinking water at or greater than the national health advisory level established from time to time by the EPA. F-54 Litigation Operating Leases Guarantees at December 31, 2018 Final Expiration Maximum Future Payments (In millions) Obligations for customers$^{1}$: Bank borrowings 2022 $ 90 Obligations for non-consolidated affiliates$^{2}$: Bank borrowings 2019 165 Residual value guarantees 3 2025 4 Total guarantees $ 259