15. Commitments and Contingencies Lease Commitments We lease certain facilities under long term operating leases. Certain of our lease agreements contain renewal options, early termination options and/or payment escalations based on fixed annual increases, local consumer price index changes or market rental reviews. We recognize rent expense with fixed rate increases and/or fixed rent reductions on a straight line basis over the term of the lease. We lease approximately 1.5 million square feet of office space in 117 locations in 54 countries. For the years ended December 31, 2017 , 2016 and 2015 , we recognized rent expense of $32 million , $26 million and $28 million , respectively. Future minimum lease payments under non-cancelable operating leases are as follows (in thousands): Amount 2018 $ 24,467 2019 20,872 2020 17,733 2021 14,189 2022 11,156 Thereafter 29,884 Total $ 118,301 Legal Proceedings While certain legal proceedings and related indemnification obligations to which we are a party specify the amounts claimed, these claims may not represent reasonably possible losses. Given the inherent uncertainties of litigation, the ultimate outcome of these matters cannot be predicted at this time, nor can the amount of possible loss or range of loss, if any, be reasonably estimated, except in circumstances where an aggregate litigation accrual has been recorded for probable and reasonably estimable loss contingencies. A determination of the amount of accrual required, if any, for these contingencies is made after careful analysis of each matter. The required accrual may change in the future due to new information or developments in each matter or changes in approach such as a change in settlement strategy in dealing with these matters. Antitrust Litigation and DOJ Investigation US Airways Antitrust Litigation In April 2011, US Airways filed suit against us in federal court in the Southern District of New York, alleging violations of the Sherman Act Section 1 (anticompetitive agreements) and Section 2 (monopolizat