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Financial Derivative Instruments - (Gain) Loss in Income by Hedging Relationship (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Interest Expense |  |
Derivative [Line Items] |  |
Gain (Loss) on Derivative Instruments, Net, Pretax | $ 37 | $ 33
Fuel |  |
Derivative [Line Items] |  |
Gain (Loss) on Derivative Instruments, Net, Pretax | (33) | 552
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
Cash Flow Hedging | Interest Expense | Interest rate derivatives |  |
Derivative [Line Items] |  |
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net | 6 | 11
Cash Flow Hedging | Fuel | Fuel derivatives |  |
Derivative [Line Items] |  |
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net | (33) | 552
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
Not Designated as Hedging Instrument | Fair Value Hedging | Interest Expense | Interest rate derivatives |  |
Derivative [Line Items] |  |
Gain (Loss) on Derivative Instruments, Net, Pretax | 8 | (1)
Not Designated as Hedging Instrument | Fair Value Hedging | Fuel | Interest rate derivatives |  |
Derivative [Line Items] |  |
Gain (Loss) on Derivative Instruments, Net, Pretax | 0 | 0
Designated as Hedging Instrument | Fair Value Hedging | Interest Expense | Interest rate derivatives |  |
Derivative [Line Items] |  |
Interest Expense, Debt | 23 | 23
Designated as Hedging Instrument | Fair Value Hedging | Fuel | Interest rate derivatives |  |
Derivative [Line Items] |  |
Interest Expense, Debt | $ 0 | $ 0

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4D -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624177-113959 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959 + 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109980867&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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6036836-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.8) -URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227 + Details
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