FAIR VALUE MEASUREMENTS (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value, by Balance Sheet Grouping [Table Text Block]

Fixed-rate debt on the consolidated balance sheet and the estimated fair value of long-term fixed-rate debt (in millions):

	December 31, 2018		December 31, 2017
Fixed rate debt at cost	$	639	$	956
Non-recurring purchase price accounting fair value adjustment	3		3
Total fixed rate debt	$	642	$	959
Estimated fair value	$	641	$	959

Fair values of financial instruments on the consolidated balance sheet (in millions):

	December 31, 2018								December 31, 2017
	Level 1		Level 2			Total			Level 1		Level 2			Total
Assets
Marketable securities
U.S. government and agency securities	$	293	$	—		$	293		$	328	$	—		$	328
Foreign government bonds	—		26			26			—		43			43
Asset-backed securities	—		190			190			—		209			209
Mortgage-backed securities	—		92			92			—		99			99
Corporate notes and bonds	—		520			520			—		726			726
Municipal securities	—		10			10			—		22			22
Derivative instruments
Fuel hedge contracts—call options	—		4			4			—		22			22
Interest rate swap agreements	—		10			10			—		9			9
Liabilities
Derivative instruments
Interest rate swap agreements	—		(7		)	(7		)	—		(8		)	(8		)

Schedule of Realized Gain (Loss) [Table Text Block]

Proceeds from sales of marketable securities were $1.1 billion, $1.4 billion and $962 million in 2018, 2017, and 2016.

Schedule of Debt Investment Maturities [Table Text Block]

Maturities for marketable securities (in millions):

December 31, 2018	Cost Basis		Fair Value
Due in one year or less	$	138	$	138
Due after one year through five years	995		981
Due after five years through 10 years	12		12
Total	$	1,145	$	1,131