Our credit rating could negatively impact our availability and cost of capital and could require us to post more collateral under certain commercial arrangements. Since December 2015, Moodyâ€™s Investor Services, Inc. and Standard & Poorâ€™s Rating Services have significantly lowered our credit ratings. The downgrades were primarily a result of the effect of low oil and natural gas prices on our ability to generate cash flow from operations. We cannot provide assurance that our credit ratings will not be reduced if commodity prices decrease. Any downgrade to our credit ratings could negatively impact our availability and cost of capital. Some of our counterparties have requested or required us to post collateral as financial assurance of our performance under certain contractual arrangements, such as gathering, transportation, processing and hedging agreements. As of February 24, 2017, we have received requests and posted approximately $275 million in collateral under such arrangements (excluding the supersedeas bond with respect to the 2019 Notes litigation discussed in Note 3 of the notes to our consolidated financial statements included in Item 8 of this report). We have posted the required collateral, primarily in the form of letters of credit and cash, or are otherwise complying with these contractual requests for collateral. We may be requested or required by other counterparties to post additional collateral in an aggregate amount of approximately $451 million (excluding the supersedeas bond with respect to the 2019 Notes litigation), which may be in the form of additional letters of credit, cash or other acceptable collateral. Any posting of collateral consisting of cash or letters of credit, which would reduce availability under our credit facility, will negatively impact our liquidity. Declines in commodity prices could result in write downs of the carrying value of our oil and natural gas properties. Under the full cost method of accounting for costs related to our oil and natural gas properties, we are required to write down the carrying value of our oil and natural gas assets if capitalized costs exceed the quarterly ceiling limit, which is based on the average of commodity prices on the first day of the month over the trailing 12-month period. Such write-downs can be material. For example, for the year ended December 31, 2016, we reported non-cash impairment charges on our oil and natural gas properties totaling $2.564 billion, primarily resulting from a substantial decrease in the trailing 12-month average first-day-of-the-month oil and natural gas prices throughout 2016. The trailing 12-month average first-day-of-the-month prices used to calculate our oil and natural gas reserves decreased from $50.28 per bbl of oil and $2.58 per mcf of natural gas as of December 31, 2015 to $42.75 per bbl of oil and $2.49 per mcf of natural gas as of December 31, 2016. As of December 31, 2016, the present value of estimated future net revenue of our proved reserves, discounted at an annual rate of 10%, was $4.4 billion. Estimated future net revenue represents the estimated future gross revenue to be generated from the production of proved reserves, net of estimated production and future development costs, using prices and costs under existing economic conditions as of that date. Further material write-downs in subsequent quarters could occur if the trailing 12-month commodity prices fall as compared to the commodity prices used as of December 31, 2016. Significant capital expenditures are required to replace our reserves and conduct our business. Our exploration, development and acquisition activities require substantial capital expenditures. We intend to fund our capital expenditures through cash flows from operations, and to the extent that is not sufficient, cash on hand and borrowings under our revolving credit facility. Our ability to generate operating cash flow is subject to a number of risks and variables, such as the level of production from existing wells, prices of oil, natural gas and NGL, our success in developing and producing new reserves and the other risk factors discussed herein. If we are unable to fund our capital expenditures as planned, we could experience a curtailment of our exploration and development activity, a loss of properties and a decline in our oil, natural gas and NGL reserves. If we are not able to replace reserves, we may not be able to sustain production. Our future success depends largely upon our ability to find, develop or acquire additional oil and natural gas reserves that are economically recoverable. Unless we replace the reserves we produce through successful development, exploration or acquisition activities, our proved reserves and production will decline over time. Our reserve estimates as of December 31, 2016, reflect an expected decline in the production rate on our producing properties of approximately 31% in 2017 and 22% in 2018. Thus, our future oil and natural gas reserves and production, and therefore our cash flow and income, are highly dependent on our success in efficiently developing our current reserves and economically finding or acquiring additional recoverable reserves.