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Financial Derivative Instruments - Fair Values by Balance Sheet Location (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | $ 283 | $ 444
Derivative Liability, Fair Value, Gross Liability | [1] | 57 | 805
Designated as Hedging Instrument |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 248 | 137
Derivative Liability, Fair Value, Gross Liability | [1] | 20 | 491
Not Designated as Hedging Instrument |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 35 | 307
Derivative Liability, Fair Value, Gross Liability | [1] | 37 | 314
Fuel derivatives | Designated as Hedging Instrument | Prepaid expenses and other current assets |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 112 | 7
Derivative Asset, Fair Value, Gross Liability | [1] | 0 | 44
Fuel derivatives | Designated as Hedging Instrument | Other Assets |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 136 | 126
Derivative Asset, Fair Value, Gross Liability | [1] | 0 | 0
Fuel derivatives | Designated as Hedging Instrument | Accrued Liabilities |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability, Fair Value, Gross Liability | [1] | 0 | 412
Derivative Liability, Fair Value, Gross Asset | [1] | 0 | 4
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] | 35 | 54
Derivative Asset, Fair Value, Gross Liability | [1] | 35 | 0
Fuel derivatives | Not Designated as Hedging Instrument | Other Assets |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 0 | 52
Derivative Asset, Fair Value, Gross Liability | [1] | 0 | 52
Fuel derivatives | Not Designated as Hedging Instrument | Accrued Liabilities |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability, Fair Value, Gross Liability | [1] | 0 | 262
Derivative Liability, Fair Value, Gross Asset | [1] | 0 | 201
Interest rate derivatives | Other Noncurrent Liabilities |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability, Fair Value, Gross Liability |  | 22 |
Interest rate derivatives | Designated as Hedging Instrument | Other Noncurrent Liabilities |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability, Fair Value, Gross Liability | [1] | 20 | 35
Derivative Liability, Fair Value, Gross Asset | [1] | 0 | 0
Interest rate derivatives | Not Designated as Hedging Instrument | Accrued Liabilities |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability, Fair Value, Gross Liability | [1] | 1 | 0
Derivative Liability, Fair Value, Gross Asset | [1] | 0 | 0
Interest rate derivatives | Not Designated as Hedging Instrument | Other Noncurrent Liabilities |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability, Fair Value, Gross Liability | [1] | 1 | 0
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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