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DERIVATIVE INSTRUMENTS - FAIR VALUE OF HEDGE POSITIONS (Details) gallons in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($) gallons aircraft agreement | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Derivative [Line Items] |  |  |
Net cash received (paid) to for new positions and settlements | $ (12) | $ (19) | $ (17)
B737-800 [Member] |  |  |
Derivative [Line Items] |  |  |
Capital Leased Assets, Number of B737-800 Aircraft | aircraft | 6 |  |
Not Designated as Hedging Instrument [Member] | Fuel hedge contracts [Member] |  |  |
Derivative [Line Items] |  |  |
Fuel hedge contracts outstanding (in gallons) | gallons | 434 |  |
Secured Debt [Member] | Interest rate swaps agreements [Member] |  |  |
Derivative [Line Items] |  |  |
Number of interest rate swap agreements | agreement | 2 |  |
Derivative, Notional Amount | $ 265 |  |
Aircraft Rent [Member] |  |  |
Derivative [Line Items] |  |  |
Price Risk Cash Flow Hedge Unrealized Gain (Loss) to be Reclassified During Next 12 Months | (3) |  |
Interest Income [Member] |  |  |
Derivative [Line Items] |  |  |
Price Risk Cash Flow Hedge Unrealized Gain (Loss) to be Reclassified During Next 12 Months | $ 1 |  |

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- Definition Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition The number of derivative instruments of a particular group held by the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5579245-113959 + Details
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- Definition The cash outflow or inflow for a financial contract that meets the hedge criteria as either a cash flow hedge, fair value hedge, or hedge of a net investment in a foreign operation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 27 -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3581-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3098-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3095-108585 + Details
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