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Financial Derivative Instruments - Offsetting of Derivative Liabilities (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Other Noncurrent Liabilities |  |  |
Offsetting Liabilities [Line Items] |  |  |
Liabilities derivative contracts, net |  | $ 21 | $ 35
Fuel derivatives | Prepaid expenses and other current assets |  |  |
Offsetting Liabilities [Line Items] |  |  |
Gross amounts of recognized liabilities |  | 50 | 48
Gross amounts offset in the Balance Sheet |  | (50) | (48)
Liabilities derivative contracts, net |  | 0 | 0
Fuel derivatives | Other Assets |  |  |
Offsetting Liabilities [Line Items] |  |  |
Gross amounts of recognized liabilities |  | 0 | 58
Gross amounts offset in the Balance Sheet |  | 0 | (58)
Liabilities derivative contracts, net | [1] | 0 | 0
Fuel derivatives | Accrued Liabilities |  |  |
Offsetting Liabilities [Line Items] |  |  |
Gross amounts of recognized liabilities |  | 0 | 674
Gross amounts offset in the Balance Sheet |  | 0 | (516)
Liabilities derivative contracts, net | [1] | 0 | 158
Interest rate derivatives | Accrued Liabilities |  |  |
Offsetting Liabilities [Line Items] |  |  |
Gross amounts of recognized liabilities |  | 1 | 0
Gross amounts offset in the Balance Sheet |  | 0 | 0
Liabilities derivative contracts, net | [1] | 1 | 0
Interest rate derivatives | Other Noncurrent Liabilities |  |  |
Offsetting Liabilities [Line Items] |  |  |
Gross amounts of recognized liabilities |  | 21 | 35
Gross amounts offset in the Balance Sheet |  | 0 | 0
Liabilities derivative contracts, net | [1] | $ 21 | $ 35
[1] | The net amounts of derivative assets and liabilities are reconciled to the individual line item amounts presented in the Consolidated Balance Sheet in Note 5.

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