COMMITMENTS AND CONTINGENCIES Lease Commitments We lease office facilities, including our headquarters and other facilities under non-cancelable operating leases. The schedule of minimum future rental payments under non-cancelable operating leases having initial terms in excess of one year at December 31, 2018 , is as follows: (in millions) Operating Leases Year ending December 31: 2019 $ 18 2020 17 2021 11 2022 9 2023 8 Thereafter 25 Minimum lease payments $ 88 The lease agreements generally provide for rental payments on a graduated basis and for options to renew, which could increase future minimum lease payments if exercised. We recognize rent expense on a straight-line basis over the lease period and accrue for rent expense incurred but not paid. Rent expense for the years ended December 31, 2018 , 2017 and 2016 was $20 million , $18 million and $17 million , respectively. Credit Facilities We maintain a $250 million revolving credit facility which includes capacity for a $20 million swingline facility. Letters of credit issued pursuant to the revolving credit facility reduce the amount available for borrowing under the revolving credit facility. The total unused portion of the revolving credit facility was $234 million as of December 31, 2018 . The terms of the credit agreement governing the revolving credit facility require us to maintain certain financial ratios at each quarter end. We were in compliance with these covenants as of December 31, 2018 . We also have a $5 million line of credit facility to secure standby letters of credit related to our workers' compensation obligation. At December 31, 2018 , the total unused portion of the credit facility was $3 million . Standby Letters of Credit We have two standby letters of credit up to an aggregate of $18 million provided as collateral for our workers compensation obligations. At December 31, 2018 , the facilities were not drawn down. Contingencies In August 2015, Howard Welgus, a purported stockholder, filed a putative securities class action lawsuit, Welgus v. TriNet Group, Inc., et. al., under the Securities Exchange Act of 1934 in the United States District Court for the Northern District of California. The complaint was later amended in April 2016 and again in March 2017. On December 18, 2017, the district court granted TriNets motion to dismiss the amended complaint in its entirety, without leave to amend. Plaintiff filed a notice of appeal of the district courts order on January 17, 2018. Plaintiff-Appellant filed his opening appeal brief before the Ninth Circuit Court of Appeals on April 27, 2018. TriNet filed a responsive brief on June 28, 2018. Plaintiff-Appellant filed his reply brief on August 20, 2018. The Ninth Circuit has scheduled a hearing date for March 14, 2019. We see no basis for a reversal of the district courts decision. We are unable to reasonably estimate the possible loss or expense, or range of losses and expenses, if any, arising from this litigation. We are and, from time to time, have been and may in the future become involved in various litigation matters, legal proceedings, and claims arising in the ordinary course of our business, including disputes with our clients or various class action, collective action, representative action, and other proceedings arising from the nature of our co-employment relationship with our clients and WSEs in which we are named as a defendant. In addition, due to the nature of our co-employment relationship with our clients and WSEs, we could be subject to liability for federal and state law violations, even if we do not participate in such violations. While our agreements with our clients contain indemnification provisions related to the conduct of our clients, we may not be able to avail ourselves of such provisions in every instance. We have accrued our current best estimates of probable losses with respect to these matters, which are individually and in aggregate immaterial to our consolidated financial statements. While the outcome of the matters described above cannot be predicted with certainty, management currently does not believe that any such claims or proceedings or the above mentioned securities class action will have a materially adverse effect on our consolidated financial position, results of operations, or cash flows. However, the unfavorable resolution of any particular matter or our reassessment of our exposure for any of the above matters based on additional information obtained in the future could have a material impact on our consolidated financial position, results of operations, or cash flows.