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%