(5) Commitments and Contingencies Lease Obligations The Company leases 260 aircraft, as well as airport facilities, office space, and various other property and equipment under non-cancelable operating leases which are generally on a long-term net rent basis where the Company pays taxes, maintenance, insurance and certain other operating expenses applicable to the leased property. The following table summarizes future minimum rental payments required under operating leases that have non-cancelable lease terms as of December 31, 2018 (in thousands): 2019 $ 87,256 2020 101,741 2021 90,787 2022 72,593 2023 65,749 Thereafter 59,820 $ 477,946 The majority of the Companys leased aircraft are owned and leased through trusts whose sole purpose is to purchase, finance and lease these aircraft to the Company (Leveraged Lease Agreements). The Company is not a beneficiary of such trusts and the Company does not have an ownership interest in such trusts. The Companys leveraged leases do not require the Company to guarantee a portion of the residual values of the leased assets held by the trust and the Companys leveraged lease agreements do not contain a fixed purchase option or have any other terms that represent variable interests in such trusts. As a result, the Company has not consolidated any of these trusts. Total rental expense for non-cancelable aircraft operating leases was approximately $154.9 million, $215.8 million and $262.6 million for the years ended December 31, 2018, 2017 and 2016, respectively. The minimum rental expense for airport station rents was approximately $19.6 million, $30.3 million and $31.4 million for the years ended December 31, 2018, 2017 and 2016, respectively. Self-insurance The Company self-insures a portion of its potential losses from claims related to workers compensation, environmental issues, property damage, medical insurance for employees and general liability. Losses are accrued based on an estimate of the ultimate aggregate liability for claims incurred, using standard industry practices and the Companys actual experience. Actual results could differ from these estimates. Legal Matters The Company is subject to certain legal actions which it considers routine to its business activities. As of December 31, 2018, management believed, after consultation with legal counsel, that the ultimate outcome of such legal matters was not likely to have a material adverse effect on the Companys financial position, liquidity or results of operations. Concentration Risk and Significant Customers The Company requires no collateral from its major airline partners or customers, but monitors the financial condition of its major airline partners. Under the majority of the Companys code-share agreements, the Company receives weekly payments from its major code-share partners that approximate a significant percentage of the compensation earned for such period. Additionally, the Company provides certain customer service functions at multiple airports for various airlines and the Company maintains an allowance for doubtful accounts receivable based upon expected collectability of all accounts receivable. The Companys allowance for doubtful accounts totaled $158,000 and $157,000 as of December 31, 2018 and 2017, respectively. For the years ended December 31, 2018, 2017 and 2016, the Companys contractual relationships with Delta and United combined accounted for approximately 81.4%, 82.9% and 88.7%, respectively of the Companys total revenues. Employees Under Collective Bargaining Agreements As of December 31, 2018, the Company had approximately 15,900 full-time equivalent employees. As of December 31, 2018, ExpressJet had 2,932 full-time equivalent employees of which approximately 2,320 employees where represented by a union. Although no SkyWest Airlines employees are represented by a union, certain SkyWest Airline employees are covered under a stable and binding collective bargaining agreement that is administered by employee representatives.