COMMITMENTS AND CONTINGENCIES Legal Matters The Company is party to proceedings and claims incidental to its business. While many of these matters involve inherent uncertainty, the Company believes that the amount of the liability, if any, ultimately incurred with respect to any such proceedings or claims will not have a material adverse effect, individually or in the aggregate, on the Companys consolidated financial position as a whole or on its liquidity, capital resources or future results of operations. The Company will continue to evaluate proceedings and claims involving the Company on a regular basis and will establish and adjust any reserves as appropriate to reflect its assessment of the then-current status of the matters. Environmental Obligations The Company is subject to various federal, state and local laws and regulations relating to the protection of the environment. These laws, which are often changing, regulate the discharge of materials into the environment and may require the Company to remove or mitigate the environmental effects of the disposal or release of petroleum or chemical substances at various sites. Environmental expenditures are expensed as incurred. The Company has established procedures for the ongoing evaluation of its operations, to identify potential environmental exposures and to comply with regulatory policies and procedures. The Company accounts for environmental contingencies in accordance with the accounting guidance related to accounting for contingencies. Environmental expenditures that relate to current operations are expensed or capitalized as appropriate. Expenditures that relate to an existing condition caused by past operations, which do not contribute to current or future revenue generation, are expensed. Liabilities are recorded when environmental assessments and/or clean-ups are probable and the costs can be reasonably estimated. Such liabilities are generally undiscounted unless the timing of cash payments is fixed and readily determinable. At December 31, 2018 and 2017 , the Company had no environmental matters requiring specific disclosure or requiring the recognition of a liability. Drilling Commitments The Company periodically enters into contractual arrangements under which the Company is committed to expend funds to drill wells in the future, including agreements to secure drilling rig services, which require the Company to make future minimum payments to the rig operators. The Company records drilling commitments in the periods in which well capital is incurred or rig services are provided. The following table summarizes the Companys drilling commitments as of December 31, 2018 (in thousands): Payments Due by Period 2019 2020 2021 2022 2023 Thereafter Total Drilling commitments $ 52,740 $ 27,754 $ 9,908 $  $  $  $ 90,402 Derivative Obligations The Companys future deferred premium payments related to derivative agreements as of December 31, 2018 was as follows (in thousands): Payments Due by Period 2019 2020 2021 2022 2023 Thereafter Total Derivative obligations $ 51,099 $ 3,285 $  $  $  $  $ 54,384 Operating Leases The Companys estimated future minimum lease payments under long-term operating lease agreements as of December 31, 2018 was as follows (in thousands): Payments Due by Period 2019 2020 2021 2022 2023 Thereafter Total Office Leases $ 9,816 $ 9,699 $ 16,639 $ 22,473 $ 21,822 $ 148,508 $ 228,957 Field Equipment 9,182 1,690 18    10,890 Office Equipment 260 260 93    613 Total $ 19,258 $ 11,649 $ 16,750 $ 22,473 $ 21,822 $ 148,508 $ 240,460 Rent expense for the years ended December 31, 2018 , 2017 and 2016 was $13.1 million , $9.5 million and $7.1 million , respectively. Firm Transportation and Crude Oil Sales Agreements The Company sells oil and natural gas under a variety of contractual agreements, some of which specify the delivery of fixed and determinable quantities. The following table presents gross volume information related to certain of the Companys long-term firm transportation and crude oil sales agreements as of December 31, 2018 : For the years ended December 31, 2019 2020 2021 2022 2023 Thereafter Total Oil (MMBbl) (1) 18.5 16.4 16.4 16.4 16.4 24.7 108.8 Dollar commitment (2) (in thousands) $ 28,937 $ 27,274 $ 27,931 $ 28,662 $ 29,401 $ 45,414 $ 187,619 (1) This table is based on third-party pipeline operations that have commenced as of December 31, 2018. See Item 1. BusinessTransportation and Delivery Commitments for additional information. (2) These amounts equal the total deficiency fees payable if the Company is unable to meet all of its contractual delivery commitments under its long-term firm transportation and crude oil sales agreements. The Company expects to fulfill these delivery commitments with production from its existing proved developed and proved undeveloped reserves, which the Company regularly monitors to ensure sufficient availability. In addition, the Company monitors its current production, its anticipated future production and its future development plans, in each case factoring in production attributable to third-party working, royalty and overriding royalty interest owners, in order to meet its delivery commitments. If production volumes are not sufficient to meet these contractual delivery commitments, the Company may be subject to deficiency fees unless it purchases commodities in the market to satisfy such commitments.