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| Debt Disclosure [Abstract] | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Debt | Debt The following table summarizes the carrying value of our outstanding debt (in millions, except percentages):
Senior Notes In 2017, we issued senior unsecured notes, or senior notes, in an aggregate principal amount of $2.5 billion. The issuance consisted of $400 million of floating rate notes due 2023, $500 million of 2.150% fixed rate notes due 2020, $750 million of 2.750% fixed rate notes due 2023 and $850 million of 3.600% fixed rate notes due 2027. In addition, $1.0 billion of 1.350% fixed rate notes due 2017 and $450 million of floating rate notes due 2017 matured and were repaid in 2017. In 2016, we issued senior unsecured notes, or senior notes, in an aggregate principal amount of $2.3 billion. The issuance consisted of $750 million aggregate principal amount of 2.500% fixed rate notes due 2018, $750 million aggregate principal amount of 3.800% fixed rate notes due 2022 and $750 million aggregate principal amount of 6.000% fixed rate notes due 2056. None of the floating rate notes are redeemable prior to maturity. On and after March 1, 2021, we may redeem some or all of the 6.000% fixed rate notes due 2056 at any time and from time to time prior to their maturity at a redemption price equal to 100% of the principal amount of the notes to be redeemed, plus accrued and unpaid interest. We may redeem some or all of the other fixed rate notes of each series at any time and from time to time prior to their maturity, generally at a make-whole redemption price, plus accrued and unpaid interest. If a change of control triggering event occurs with respect to the 2.500% fixed rate notes due 2018, the 2.150% fixed rate notes due 2020, the 3.800% fixed rate notes due 2022, the floating rate notes due 2023, the 2.750% fixed rate notes due 2023, the 3.600% fixed rate notes due 2027 or the 6.000% fixed rate notes due 2056, we must, subject to certain exceptions, offer to repurchase all of the notes of the applicable series at a price equal to 101% of the principal amount, plus accrued and unpaid interest. The indenture pursuant to which the senior notes were issued includes customary covenants that, among other things and subject to exceptions, limit our ability to incur, assume or guarantee debt secured by liens on specified assets or enter into sale and lease-back transactions with respect to specified properties, and also includes customary events of default. To help achieve our interest rate risk management objectives, in connection with the previous issuance of certain senior notes, we entered into interest rate swap agreements that effectively converted $2.4 billion of our fixed rate notes to floating rate debt based on LIBOR plus a spread. These swaps were designated as fair value hedges against changes in the fair value of certain fixed rate senior notes resulting from changes in interest rates. The gains and losses related to changes in the fair value of interest rate swaps substantially offset changes in the fair value of the hedged portion of the underlying debt that are attributable to changes in market interest rates. The effective interest rates for our senior notes include the interest payable, the amortization of debt issuance costs and the amortization of any original issue discount on these senior notes. Interest on these senior notes is payable either quarterly or semiannually. Interest expense associated with these senior notes, including amortization of debt issuance costs, during the years ended December 31, 2017, 2016 and 2015 was approximately $307 million, $254 million and $178 million, respectively. As of December 31, 2017 and 2016, the estimated fair value of these senior notes, using Level 2 inputs, was approximately $10.1 billion and $8.9 billion, respectively. Commercial Paper We have an up to $1.5 billion commercial paper program pursuant to which we may issue commercial paper notes with maturities of up to 397 days from the date of issue in an aggregate principal amount at maturity of up to $1.5 billion outstanding at any time outstanding. As of December 31, 2017, there were no commercial paper notes outstanding. Credit Agreement In November 2015, we entered into a credit agreement that provides for an unsecured $2 billion five-year revolving credit facility. We may also, subject to the agreement of the applicable lenders, increase the commitments under the revolving credit facility by up to an aggregate amount of $1 billion. Funds borrowed under the credit agreement may be used for working capital, capital expenditures, acquisitions and other general corporate purposes. As of December 31, 2017, no borrowings were outstanding under our $2 billion credit agreement. However, as described above, we have an up to $1.5 billion commercial paper program and therefore maintain $1.5 billion of available borrowing capacity under our credit agreement in order to repay commercial paper borrowings in the event we are unable to repay those borrowings from other sources when they become due. As a result, $500 million of borrowing capacity was available as of December 31, 2017 for other purposes permitted by the credit agreement. Loans under the credit agreement bear interest at either (i) LIBOR plus a margin (based on our public debt credit ratings) ranging from 0.875 percent to 1.5 percent or (ii) a formula based on the agent bank’s prime rate, the federal funds effective rate plus 0.5 percent or LIBOR plus 1.0 percent, plus a margin (based on our public debt credit ratings) ranging from zero percent to 0.5 percent. The credit agreement will terminate and all amounts owing thereunder will be due and payable on November 9, 2020, unless (a) the commitments are terminated earlier, either at our request or, if an event of default occurs, by the lenders (or automatically in the case of certain bankruptcy-related events of default), or (b) the maturity date is extended upon our request, subject to the agreement of the lenders. The credit agreement includes customary representations, warranties, affirmative and negative covenants, including financial covenants, events of default and indemnification provisions in favor of the banks. The negative covenants include restrictions regarding the incurrence of liens and subsidiary indebtedness, in each case, subject to certain exceptions. The financial covenants require us to meet a quarterly financial test with respect to a minimum consolidated interest coverage ratio and a maximum consolidated leverage ratio. The events of default include the occurrence of a change of control (as defined in the credit agreement) with respect to us. We were in compliance with all covenants in our outstanding debt instruments for the period ended December 31, 2017. Future Maturities Expected future principal maturities as of December 31, 2017 are as follows (in millions):
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