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Financial Derivative Instruments - Fair Values by Balance Sheet Location (Details) - USD ($) $ in Millions | Jun. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Derivatives, Fair Value [Line Items] |  |
Gross amounts of recognized assets | $ 503 | $ 134
Gross amounts of recognized liabilities | 3 | 6
Fuel derivatives | Prepaid expenses and other current assets |  |
Derivatives, Fair Value [Line Items] |  |
Gross amounts of recognized assets | 224 | 13
Gross amounts offset in the Balance Sheet | 48 | 3
Gross amounts offset in the Balance Sheet | 48 | 3
Gross amounts of recognized liabilities | 48 | 3
Fuel derivatives | Other Current Assets |  |
Derivatives, Fair Value [Line Items] |  |
Cash collateral deposits held from counterparties for fuel contracts - current | 48 | 3
Fuel derivatives | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Gross amounts of recognized assets | 278 | 121
Gross amounts offset in the Balance Sheet | 72 | 31
Gross amounts offset in the Balance Sheet | 72 | 31
Gross amounts of recognized liabilities | 72 | 31
Cash collateral deposits held from counterparties for fuel contracts - current | 72 | 31
Interest rate derivatives | Other noncurrent liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Gross amounts offset in the Balance Sheet | 0 | 0
Gross amounts of recognized liabilities | 3 | 6
Interest rate derivatives | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Gross amounts of recognized assets | 1 | 0
Gross amounts offset in the Balance Sheet | 0 | 0
Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Gross amounts of recognized assets | 483 | 130
Gross amounts of recognized liabilities | 3 | 6
Designated as Hedging Instrument | Fuel derivatives | Prepaid expenses and other current assets |  |
Derivatives, Fair Value [Line Items] |  |
Gross amounts of recognized assets | 204 | 9
Designated as Hedging Instrument | Fuel derivatives | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Gross amounts of recognized assets | 278 | 121
Designated as Hedging Instrument | Interest rate derivatives | Other noncurrent liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Gross amounts of recognized liabilities | 3 | 6
Designated as Hedging Instrument | Interest rate derivatives | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Gross amounts of recognized assets | 1 |
Not Designated as Hedging Instrument | Fuel derivatives | Prepaid expenses and other current assets |  |
Derivatives, Fair Value [Line Items] |  |
Gross amounts of recognized assets | $ 20 | $ 4

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