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FAIR VALUE MEASUREMENTS (Tables) | 12 Months Ended
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Dec. 31, 2024
Fair Value Disclosures [Abstract] |
Fair Value, by Balance Sheet Grouping | Fair values of financial instruments on the consolidated balance sheet:
| December 31, 2024
(in millions) | Level 1 |  | Level 2 |  | Level 3 |  | Total
Assets |  |  |  |  |  |  |
Marketable securities |  |  |  |  |  |  |
U.S. government and agency securities | $ | 292 |  |  | $ | — |  |  | $ | — |  |  | $ | 292 |
Equity mutual funds | 7 |  |  | — |  |  | — |  |  | 7 |
Asset-backed securities | — |  |  | 127 |  |  | 7 |  |  | 134 |
Mortgage-backed securities | — |  |  | 112 |  |  | — |  |  | 112 |
Corporate notes and bonds | — |  |  | 696 |  |  | 2 |  |  | 698 |
Municipal securities and other | — |  |  | 31 |  |  | — |  |  | 31 |
Total Marketable securities | 299 |  |  | 966 |  |  | 9 |  |  | 1,274 |
Derivative instruments |  |  |  |  |  |  |
Interest rate swap agreements | — |  |  | 9 |  |  | — |  |  | 9 |
Total Assets | $ | 299 |  |  | $ | 975 |  |  | $ | 9 |  |  | $ | 1,283 |
| December 31, 2023
(in millions) | Level 1 |  | Level 2 |  | Level 3 |  | Total
Assets |  |  |  |  |  |  |
Marketable securities |  |  |  |  |  |  |
U.S. government and agency securities | $ | 387 |  |  | $ | — |  |  | $ | — |  |  | $ | 387 |
Equity mutual funds | 5 |  |  | — |  |  | — |  |  | 5 |
Foreign government bonds | — |  |  | 10 |  |  | — |  |  | 10 |
Asset-backed securities | — |  |  | 192 |  |  | — |  |  | 192 |
Mortgage-backed securities | — |  |  | 115 |  |  | — |  |  | 115 |
Corporate notes and bonds | — |  |  | 763 |  |  | — |  |  | 763 |
Municipal securities and other | — |  |  | 38 |  |  | — |  |  | 38 |
Total Marketable securities | 392 |  |  | 1,118 |  |  | — |  |  | 1,510
Derivative instruments |  |  |  |  |  |  |
Fuel hedge contracts - call options | — |  |  | 11 |  |  | — |  |  | 11 |
Interest rate swap agreements | — |  |  | 8 |  |  | — |  |  | 8 |
Total Assets | $ | 392 |  |  | $ | 1,137 |  |  | $ | — |  |  | $ | 1,529 |
Fixed-rate debt on the consolidated balance sheet and the estimated fair value of long-term fixed-rate debt:
(in millions) | December 31, 2024 |  | December 31, 2023
Fixed-rate debt | $ | 2,946 |  |  | $ | 1,515 |
Estimated fair value | $ | 2,844 |  |  | $ | 1,382 |
Schedule of Debt Investment Maturities | Maturities for marketable securities:
December 31, 2024 (in millions) | Cost Basis |  | Fair Value
Due in one year or less | $ | 461 |  |  | $ | 460 |
Due after one year through five years | 720 |  |  | 705 |
Due after five years through ten years | 95 |  |  | 92 |
Due after ten years | 10 |  |  | 9 |
No maturity date | 5 |  |  | 8 |
Total | $ | 1,291 |  |  | $ | 1,274 |

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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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