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Financial Derivative Instruments - Fair Values by Balance Sheet Location (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | $ 698 | $ 283
Derivative Liability, Fair Value, Gross Liability | [1] | 22 | 57
Designated as Hedging Instrument |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 698 | 248
Derivative Liability, Fair Value, Gross Liability | [1] | 22 | 20
Not Designated as Hedging Instrument |  |  |
Derivatives, Fair Value [Line Items] |  |  |
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 [Line Items] |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 348 | 112
Derivative Asset, Fair Value, Gross Liability | [1] | 0 | 0
Fuel derivatives | Designated as Hedging Instrument | Other Assets |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 350 | 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 [Line Items] |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 0 | 35
Derivative Asset, Fair Value, Gross Liability | [1] | 0 | 35
Interest rate derivatives | Designated as Hedging Instrument | Other Noncurrent Liabilities |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability, Fair Value, Gross Asset | [1] | 0 | 0
Derivative Liability, Fair Value, Gross Liability | [1] | 22 | 20
Interest rate derivatives | Not Designated as Hedging Instrument | Other Noncurrent Liabilities |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability, Fair Value, Gross Asset | [1] | 0 | 0
Derivative Liability, Fair Value, Gross Liability | [1] | 0 | 1
Interest rate derivatives | Not Designated as Hedging Instrument | Accrued Liabilities [Member] |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability, Fair Value, Gross Asset | [1] | 0 | 0
Derivative Liability, Fair Value, Gross Liability | [1] | $ 0 | $ 1
[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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