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