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Financial Derivative Instruments - (Gain) Loss in Income by Hedging Relationship (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Fuel |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (Loss) on Derivative Instruments, Net, Pretax | $ (16) | $ 157 | $ (12) | $ 296
Interest Expense |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (Loss) on Derivative Instruments, Net, Pretax | 9 | 8 | 19 | 16
Cash Flow Hedging | Fuel | Fuel derivatives |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net | (16) | 157 | (12) | 296
Cash Flow Hedging | Fuel | Interest rate derivatives |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net | 0 | 0 | 0 | 0
Cash Flow Hedging | Interest Expense | Fuel derivatives |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net | 0 | 0 | 0 | 0
Cash Flow Hedging | Interest Expense | Interest rate derivatives |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net | 1 | 3 | 3 | 6
Designated as Hedging Instrument | Fair Value Hedging | Fuel | Interest rate derivatives |  |  |  |
Derivative [Line Items] |  |  |  |
Interest Expense, Debt | 0 | 0 | 0 | 0
Designated as Hedging Instrument | Fair Value Hedging | Interest Expense | Interest rate derivatives |  |  |  |
Derivative [Line Items] |  |  |  |
Interest Expense, Debt | 6 | 6 | 12 | 12
Not Designated as Hedging Instrument | Fair Value Hedging | Fuel | Interest rate derivatives |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (Loss) on Derivative Instruments, Net, Pretax | 0 | 0 | 0 | 0
Not Designated as Hedging Instrument | Fair Value Hedging | Interest Expense | Interest rate derivatives |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (Loss) on Derivative Instruments, Net, Pretax | $ 2 | $ (1) | $ 4 | $ (2)

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- Definition The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4D -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624177-113959 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5618551-113959 + Details
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- Definition Amount of the cost of borrowed funds accounted for as interest expense for debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.8) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
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