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Debt
 12 Months Ended
Dec. 31, 2023
Debt Disclosure [Abstract]  
Debt Debt
Short-Term Debt
We have a debt financing program of up to $10.0 billion through the issuance of commercial paper. Net proceeds from this program are used for general corporate purposes. We had no commercial paper outstanding as of December 31, 2022 and 2023.
Our short-term debt balance also includes the current portion of certain long-term debt.
Long-Term Debt
Total outstanding debt is summarized below (in millions, except percentages):
Effective Interest RateAs of December 31,
MaturityCoupon Rate20222023
Debt
2014-2020 Notes issuances2024 - 2060
0.45% - 3.38%
0.57% - 3.38%
$13,000 $13,000 
Future finance lease payments, net and other (1)
2,142 1,746 
      Total debt15,142 14,746 
Unamortized discount and debt issuance costs(143)(130)
Less: Current portion of long-term notes(2)
0 (1,000)
Less: Current portion of future finance lease payments, net and other current debt(1)(2)
(298)(363)
       Total long-term debt$14,701 $13,253 
(1)Future finance lease payments are net of imputed interest.
(2)Total current portion of long-term debt is included within other accrued expenses and current liabilities. See Note 7 for further details.
The notes in the table above are fixed-rate senior unsecured obligations and generally rank equally with each other. We may redeem the notes at any time in whole or in part at specified redemption prices. The effective interest rates are based on proceeds received with interest payable semi-annually.
The total estimated fair value of the outstanding notes was approximately $9.9 billion and $10.3 billion as of December 31, 2022 and December 31, 2023, respectively. The fair value was determined based on observable market prices of identical instruments in less active markets and is categorized accordingly as Level 2 in the fair value hierarchy.
As of December 31, 2023, the aggregate future principal payments for long-term debt, including finance lease liabilities, for each of the next five years and thereafter were as follows (in millions):
2024$1,299 
20251,163
20262,165
20271,143
2028132
Thereafter8,960
Total$14,862 
Credit Facility
As of December 31, 2023, we had $10.0 billion of revolving credit facilities, of which $4.0 billion expires in April 2024 and $6.0 billion expires in April 2028. The interest rates for all credit facilities are determined based on a formula using certain market rates, as well as our progress toward the achievement of certain sustainability goals. No amounts were outstanding under the credit facilities as of December 31, 2022 and 2023.