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Financial Derivative Instruments - Fair Values by Balance Sheet Location (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Derivatives, Fair Value |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | $ 138 | $ 283
Derivative Liability, Fair Value, Gross Liability | [1] | 14 | 57
Designated as Hedging Instrument |  |  |
Derivatives, Fair Value |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 138 | 248
Derivative Liability, Fair Value, Gross Liability | [1] | 14 | 20
Not Designated as Hedging Instrument |  |  |
Derivatives, Fair Value |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 0 | 35
Derivative Liability, Fair Value, Gross Liability | [1] | 0 | 37
Fuel derivatives | Designated as Hedging Instrument | Prepaid expenses and other current assets |  |  |
Derivatives, Fair Value |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 43 | 112
Derivative Asset, Fair Value, Gross Liability | [1] | 0 | 0
Fuel derivatives | Designated as Hedging Instrument | Other Assets |  |  |
Derivatives, Fair Value |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 95 | 136
Derivative Asset, Fair Value, Gross Liability | [1] | 0 | 0
Fuel derivatives | Not Designated as Hedging Instrument | Prepaid expenses and other current assets |  |  |
Derivatives, Fair Value |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 0 | 35
Derivative Asset, Fair Value, Gross Liability | [1] | 0 | 35
Interest rate derivatives | Other Noncurrent Liabilities |  |  |
Derivatives, Fair Value |  |  |
Derivative Liability, Fair Value, Gross Liability |  | 14 |
Interest rate derivatives | Designated as Hedging Instrument | Accrued Liabilities |  |  |
Derivatives, Fair Value |  |  |
Derivative Liability, Fair Value, Gross Liability | [1] | 2 | 0
Derivative Liability, Fair Value, Gross Asset | [1] | 0 | 0
Interest rate derivatives | Designated as Hedging Instrument | Other Noncurrent Liabilities |  |  |
Derivatives, Fair Value |  |  |
Derivative Liability, Fair Value, Gross Liability | [1] | 12 | 20
Derivative Liability, Fair Value, Gross Asset | [1] | 0 | 0
Interest rate derivatives | Not Designated as Hedging Instrument | Accrued Liabilities |  |  |
Derivatives, Fair Value |  |  |
Derivative Liability, Fair Value, Gross Liability | [1] | 0 | 1
Derivative Liability, Fair Value, Gross Asset | [1] | 0 | 0
Interest rate derivatives | Not Designated as Hedging Instrument | Other Noncurrent Liabilities |  |  |
Derivatives, Fair Value |  |  |
Derivative Liability, Fair Value, Gross Liability | [1] | 0 | 1
Derivative Liability, Fair Value, Gross Asset | [1] | $ 0 | $ 0
[1] | Represents the position of each trade before consideration of offsetting positions with each counterparty and does not include the impact of cash collateral deposits provided to or received from counterparties. See discussion of credit risk and collateral following in this Note.

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