The following table summarizes our aggregate liabilities and commitments by year of maturity: Future interest costs are based upon the effective interest rates of our outstanding senior notes and the December 31, 2013 rate for our bank credit facility. We have obligations to incur future payments for dismantlement, abandonment and restoration costs of oil and gas properties. These payments are currently estimated to be incurred primarily after 2018. We record a separate liability for the fair value of these asset retirement obligations, which totaled $14.5 million as of December 31, 2013. 2014 2015 2016 2017 2018 Thereafter Total (In thousands) Bank credit facility .......... $ — $ — $ — $ — $ 210,000 $ — $ 210,000 7$^{3}$/$_{4}$% senior notes .......... — — — — — 300,000 300,000 9$^{1}$/$_{2}$% senior notes .......... — — — — — 300,000 300,000 Interest on debt ............. 55,782 55,782 55,782 55,782 55,356 47,376 325,860 Operating leases ............ 1,955 1,994 1,993 2,021 2,060 4,680 14,703 Natural gas transportation agreements .............. 7,874 5,112 2,201 1,781 1,697 691 19,356 Contracted drilling services . . . 31,136 13,754 — — — — 44,890 $ 96,747 $ 76,642 $ 59,976 $ 59,584 $ 269,113 $ 652,747 $1,214,809 Federal Taxation At December 31, 2013 we had U.S. federal net operating loss carryforwards of approximately $80.2 million and Louisiana state net operating loss carryforwards of approximately $810.6 million. Utilization of $34.7 million of our U.S. federal net operating loss carryforwards is limited to approximately $1.1 million per year pursuant to a prior change of control of an acquired company and a valuation allowance of $23.0 million, with a tax effect of $8.0 million, has been established for the estimated U.S. federal net operating loss carryforwards that will not be utilized. Realization of the remaining U.S. federal net operating loss carryforwards requires Comstock to generate taxable income within the carryforward period. A valuation allowance of $528.1 million, with a tax effect of $27.5 million, has been established against our Louisiana state net operating loss carryforwards due to the uncertainty of generating taxable income in the state of Louisiana prior to the expiration of the carryforward period. Our federal income tax returns for the years subsequent to December 31, 2009 remain subject to examination. Our income tax returns in major state income tax jurisdictions remain subject to examination for various periods subsequent to December 31, 2008. We currently believe that our significant filing positions are highly certain and that all of our significant income tax filing positions and deductions would be sustained upon audit or the final resolution would not have a material effect on our consolidated financial statements. Therefore, we have not established any significant reserves for uncertain tax positions. Interest and penalties resulting from audits by tax authorities have been immaterial and are included in the provision for income taxes in the consolidated statements of operations. The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires us to make estimates and use assumptions that can affect the reported amounts of assets, liabilities, revenues or expenses. Critical Accounting Policies 55