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Financial Derivative Instruments - Offsetting of Derivative Liabilities (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Fuel derivatives | Prepaid expenses and other current assets |  |  |
Offsetting Liabilities [Line Items] |  |  |
Gross amounts of recognized liabilities |  | $ 148 | $ 50
Gross asset amounts offset in the Balance Sheet |  | (148) | (50)
Liability derivative contracts, net |  | 0 | 0
Fuel derivatives | Other Assets |  |  |
Offsetting Liabilities [Line Items] |  |  |
Gross amounts of recognized liabilities |  | 17 | 0
Gross asset amounts offset in the Balance Sheet |  | (17) | 0
Liability derivative contracts, net | [1] | 0 | 0
Interest rate derivatives | Accrued Liabilities [Member] |  |  |
Offsetting Liabilities [Line Items] |  |  |
Gross amounts of recognized liabilities |  | 0 | 1
Gross asset amounts offset in the Balance Sheet |  | 0 | 0
Liability derivative contracts, net |  | 0 | 1
Interest rate derivatives | Other Noncurrent Liabilities |  |  |
Offsetting Liabilities [Line Items] |  |  |
Gross amounts of recognized liabilities |  | 22 | 21
Gross asset amounts offset in the Balance Sheet |  | 0 | 0
Liability derivative contracts, net | [1] | $ 22 | $ 21
[1] | The net amounts of derivative assets and liabilities are reconciled to the individual line item amounts presented in the unaudited Condensed Consolidated Balance Sheet in Note 9.

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