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Financial Derivative Instruments - Fair Values by Balance Sheet Location (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | $ 234 | $ 444
Derivative Liability, Fair Value, Gross Liability | [1] | 265 | 805
Designated as Hedging Instrument |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 124 | 137
Derivative Liability, Fair Value, Gross Liability | [1] | 138 | 491
Not Designated as Hedging Instrument |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 110 | 307
Derivative Liability, Fair Value, Gross Liability | [1] | 127 | 314
Fuel derivatives |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Collateral Already Posted, Aggregate Fair Value |  | 14 |
Fuel derivatives | Prepaid expenses and other current assets |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Total Collateral Already Received Aggregate Fair Value |  | 3 | 4
Fuel derivatives | Accrued Liabilities |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Collateral Already Posted, Aggregate Fair Value |  | 18 | 311
Fuel derivatives | Accounts Payable |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Due To Third Parties For Settled Fuel Contracts |  | 50 | 75
Fuel derivatives | Other Assets |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Total Collateral Already Received Aggregate Fair Value |  | 1 | 6
Fuel derivatives | Designated as Hedging Instrument | Prepaid expenses and other current assets |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 23 | 7
Derivative Asset, Fair Value, Gross Liability | [1] | 36 | 44
Fuel derivatives | Designated as Hedging Instrument | Accrued Liabilities |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability, Fair Value, Gross Asset | [1] | 16 | 4
Derivative Liability, Fair Value, Gross Liability | [1] | 84 | 412
Fuel derivatives | Designated as Hedging Instrument | Other Assets |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 85 | 126
Derivative Asset, Fair Value, Gross Liability | [1] | 0 | 0
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] | 41 | 54
Derivative Asset, Fair Value, Gross Liability | [1] | 25 | 0
Fuel derivatives | Not Designated as Hedging Instrument | Accrued Liabilities |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability, Fair Value, Gross Asset | [1] | 55 | 201
Derivative Liability, Fair Value, Gross Liability | [1] | 85 | 262
Fuel derivatives | Not Designated as Hedging Instrument | Other Assets |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Asset, Fair Value, Gross Asset | [1] | 14 | 52
Derivative Asset, Fair Value, Gross Liability | [1] | 14 | 52
Interest rate derivatives | Designated as Hedging Instrument | Other Noncurrent Liabilities |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability, Fair Value, Gross Asset | [1] | 0 | 0
Derivative Liability, Fair Value, Gross Liability | [1] | 18 | 35
Interest rate derivatives | Not Designated as Hedging Instrument | Other Noncurrent Liabilities |  |  |
Derivatives, Fair Value [Line Items] |  |  |
Derivative Liability, Fair Value, Gross Asset | [1] | 0 | 0
Derivative Liability, Fair Value, Gross Liability | [1] | $ 3 | $ 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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- 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 Total cash, current and noncurrent, that is already received, at the end of the reporting period, as collateral for derivative instruments. + 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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- Definition Fair value of liability associated with 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226003-175313 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959 + Details
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