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FAIR VALUE MEASUREMENTS (Tables) | 3 Months Ended
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Mar. 31, 2024
Fair Value Disclosures [Abstract] |
Investments Classified by Contractual Maturity Date | Maturities for marketable securities (in millions):
March 31, 2024 | Cost Basis |  | Fair Value
Due in one year or less | $ | 443 |  |  | $ | 436 |
Due after one year through five years | 939 |  |  | 907 |
Due after five years | 34 |  |  | 31 |
Due after 10 years | 14 |  |  | 13 |
No maturity date | 4 |  |  | 6 |
Total | $ | 1,434 |  |  | $ | 1,393 |
Fair Value, by Balance Sheet Grouping | Fair values of financial instruments on the condensed consolidated balance sheets (in millions):
| March 31, 2024 |  | December 31, 2023
| Level 1 |  | Level 2 |  | Total |  | Level 1 |  | Level 2 |  | Total
Assets |  |  |  |  |  |  |  |  |  |  |
Marketable securities |  |  |  |  |  |  |  |  |  |  |
U.S. government and agency securities | $ | 370 |  |  | $ | — |  |  | $ | 370 |  |  | $ | 387 |  |  | $ | — |  |  | $ | 387 |
Equity mutual funds | 6 |  |  | — |  |  | 6 |  |  | 5 |  |  | — |  |  | 5 |
Foreign government bonds | — |  |  | 10 |  |  | 10 |  |  | — |  |  | 10 |  |  | 10 |
Asset-backed securities | — |  |  | 170 |  |  | 170 |  |  | — |  |  | 192 |  |  | 192 |
Mortgage-backed securities | — |  |  | 103 |  |  | 103 |  |  | — |  |  | 115 |  |  | 115 |
Corporate notes and bonds | — |  |  | 695 |  |  | 695 |  |  | — |  |  | 763 |  |  | 763 |
Municipal securities | — |  |  | 39 |  |  | 39 |  |  | — |  |  | 38 |  |  | 38 |
Total Marketable securities | 376 |  |  | 1,017 |  |  | 1,393 |  |  | 392 |  |  | 1,118 |  |  | 1,510 |
Derivative instruments |  |  |  |  |  |  |  |  |  |  |
Fuel hedge contracts - call options | — |  |  | 11 |  |  | 11 |  |  | — |  |  | 11 |  |  | 11 |
Interest rate swap agreements | — |  |  | 9 |  |  | 9 |  |  | — |  |  | 8 |  |  | 8 |
Total Assets | $ | 376 |  |  | $ | 1,037 |  |  | $ | 1,413 |  |  | $ | 392 |  |  | $ | 1,137 |  |  | $ | 1,529 |
Fixed-rate debt on the condensed consolidated balance sheets and the estimated fair value of long-term fixed-rate debt (in millions):
| March 31, 2024 |  | December 31, 2023
Fixed-rate debt | $ | 1,452 |  |  | $ | 1,515 |
Estimated fair value | $ | 1,346 |  |  | $ | 1,382 |

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- Definition Tabular disclosure of the fair value of financial instruments, including financial assets and financial liabilities, and the measurements of those instruments, assets, and liabilities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 11 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482907/825-10-50-11 + Details
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- Definition Tabular disclosure of maturities of an entity's investments as well as any other information pertinent to the investments. + References No definition available. + Details
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