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Financial Derivative Instruments - Offsetting of Derivative Assets (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Offsetting Assets [Line Items] |  |  |
Asset derivative contracts, net |  | $ 136 | $ 120
Fuel derivatives | Prepaid expenses and other current assets |  |  |
Offsetting Assets [Line Items] |  |  |
Gross amounts of recognized assets |  | 147 | 61
Gross amounts offset in the Balance Sheet |  | (50) | (48)
Asset derivative contracts, net |  | 97 | 13
Fuel derivatives | Other Assets |  |  |
Offsetting Assets [Line Items] |  |  |
Gross amounts of recognized assets |  | 136 | 178
Gross amounts offset in the Balance Sheet |  | 0 | (58)
Asset derivative contracts, net | [1] | 136 | 120
Fuel derivatives | Accrued Liabilities |  |  |
Offsetting Assets [Line Items] |  |  |
Gross amounts of recognized assets |  | 0 | 516
Gross amounts offset in the Balance Sheet |  | 0 | (516)
Asset derivative contracts, net | [1] | $ 0 | $ 0
[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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- Definition Net fair value of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected to be offset against an obligation to return collateral. + References No definition available. + Details
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- Definition Fair value of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, not subject to a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 22 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226052-175313 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 + Details
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