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Financial Derivative Instruments - Collateral by Balance Sheet Location (Details) - Fuel derivatives - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Derivative [Line Items] |  |
Collateral Already Posted, Aggregate Fair Value | $ 15 |
Prepaid expenses and other current assets |  |
Derivative [Line Items] |  |
Total Collateral Already Received Aggregate Fair Value | 15 | $ 4
Other Assets |  |
Derivative [Line Items] |  |
Total Collateral Already Received Aggregate Fair Value | 0 | 6
Accounts Payable |  |
Derivative [Line Items] |  |
Due To Third Parties For Settled Fuel Contracts | 29 | 75
Accrued Liabilities |  |
Derivative [Line Items] |  |
Collateral Already Posted, Aggregate Fair Value | $ 0 | $ 311

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- Definition Amount due to third parties for settled fuel contracts as of the end of the period reported, recorded in Accrued liabilities on the balance sheet. + References No definition available. + Details
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- Definition The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624258-113959 + Details
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