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GENERAL AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - PROPERTY, EQUIPMENT AND DEPRECIATION (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2016 | Dec. 31, 2016 | Dec. 31, 2015
Property, Plant and Equipment [Line Items] |  |  |
Goodwill |  | $ 1,934 | $ 0
Buildings [Member] | Maximum [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Estimated useful life |  | 30 years |
Buildings [Member] | Minimum [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Estimated useful life |  | 25 years |
Minor building and land improvements [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Estimated useful life |  | 10 years |
Computer hardware and software [Member] | Maximum [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Estimated useful life |  | 5 years |
Computer hardware and software [Member] | Minimum [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Estimated useful life |  | 3 years |
Other furniture and equipment [Member] | Maximum [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Estimated useful life |  | 10 years |
Other furniture and equipment [Member] | Minimum [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Estimated useful life |  | 5 years |
B737 | Aircraft [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Estimated useful life | 20 years | 25 years |
B737 | Aircraft [Member] | Maximum [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Estimated useful life |  | 25 years |
B737 | 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 |
Airbus [Member] | Aircraft [Member] | Minimum [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Estimated useful life |  | 20 years |
Q400 [Member] | Aircraft [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Estimated useful life |  | 15 years |
Service Life [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Depreciation, decrease |  | $ 17 |

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