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GENERAL AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - PROPERTY, EQUIPMENT AND DEPRECIATION (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Property, Plant and Equipment [Line Items] |  |
Goodwill | $ 1,943 | $ 1,934
Buildings [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, Plant, and Equipment, Salvage Value, Percentage | 0.00% |
Buildings [Member] | Minimum [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Estimated useful life | 25 years |
Buildings [Member] | Maximum [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Estimated useful life | 30 years |
Minor building and land improvements [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Estimated useful life | 10 years |
Property, Plant, and Equipment, Salvage Value, Percentage | 0.00% |
Leaseholds and Leasehold Improvements [Member] | Minimum [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, Plant, and Equipment, Salvage Value, Percentage | 0.00% |
Leaseholds and Leasehold Improvements [Member] | Maximum [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, Plant, and Equipment, Salvage Value, Percentage | 10.00% |
Computer hardware and software [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, Plant, and Equipment, Salvage Value, Percentage | 0.00% |
Computer hardware and software [Member] | Minimum [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Estimated useful life | 3 years |
Computer hardware and software [Member] | Maximum [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Estimated useful life | 10 years |
Other furniture and equipment [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, Plant, and Equipment, Salvage Value, Percentage | 0.00% |
Other furniture and equipment [Member] | Minimum [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Estimated useful life | 5 years |
Other furniture and equipment [Member] | Maximum [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Estimated useful life | 10 years |
B737 | Aircraft [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, Plant, and Equipment, Salvage Value, Percentage | 10.00% |
B737 | Aircraft [Member] | Minimum [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Estimated useful life | 20 years |
B737 | Aircraft [Member] | Maximum [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Estimated useful life | 25 years |
Airbus [Member] | Aircraft [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, Plant, and Equipment, Salvage Value, Percentage | 10.00% |
Airbus [Member] | Aircraft [Member] | Minimum [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Estimated useful life | 20 years |
Airbus [Member] | Aircraft [Member] | Maximum [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Estimated useful life | 25 years |
E175 [Member] | Aircraft [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Property, Plant, and Equipment, Salvage Value, Percentage | 10.00% |
E175 [Member] | Aircraft [Member] | Minimum [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Estimated useful life | 20 years |
E175 [Member] | Aircraft [Member] | Maximum [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Estimated useful life | 25 years |
Q400 [Member] | Aircraft [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Estimated useful life | 15 years |
Property, Plant, and Equipment, Salvage Value, Percentage | 10.00% |

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- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=64834752&loc=SL49117168-202975 + Details
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