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Investments and Fair Value Measurements - Narrative (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2019 USD ($) | Dec. 31, 2020 USD ($) aircraft | Dec. 31, 2019 USD ($)
Fair Value [Line Items] |  |  |
Unrealized gain (loss) on derivative asset |  | $ (24) | $ 13
Investments in securities accounted for under the equity method |  | $ 139 |
Republic |  |  |
Fair Value [Line Items] |  |  |
Number of regional aircraft | aircraft |  | 66 |
Champlain |  |  |
Fair Value [Line Items] |  |  |
Number of regional aircraft | aircraft |  | 49 |
Asset-backed Securities | Minimum |  |  |
Fair Value [Line Items] |  |  |
Available-for-sale securities remaining maturities |  | 1 year |
Asset-backed Securities | Maximum |  |  |
Fair Value [Line Items] |  |  |
Available-for-sale securities remaining maturities |  | 14 years |
Corporate Debt |  |  |
Fair Value [Line Items] |  |  |
Available-for-sale securities remaining maturities |  | 2 years |
U.S. Government and Other Securities |  |  |
Fair Value [Line Items] |  |  |
Available-for-sale securities remaining maturities |  | 1 year |
Azul |  |  |
Fair Value [Line Items] |  |  |
Equity stake |  | 2.00% |
Gain (loss) on equity securities | $ (136) | $ 180 |
Carrying value of investment |  | $ 205 |
Azul | Preferred Stock |  |  |
Fair Value [Line Items] |  |  |
Equity stake |  | 8.00% |
Republic |  |  |
Fair Value [Line Items] |  |  |
Ownership stake |  | 19.00% |
ManaAir |  |  |
Fair Value [Line Items] |  |  |
Ownership stake |  | 49.90% |
Champlain |  |  |
Fair Value [Line Items] |  |  |
Ownership stake |  | 40.00% |
Fulcrum |  |  |
Fair Value [Line Items] |  |  |
Ownership interest |  | 0.06 |
Carrying value of other investment |  | $ 51 |
Fulcrum | Preferred Stock |  |  |
Fair Value [Line Items] |  |  |
Ownership interest |  | 0.07 |
CLEAR |  |  |
Fair Value [Line Items] |  |  |
Ownership interest |  | 0.01 |
Carrying value of other investment |  | $ 9 |

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