FAIR VALUE MEASUREMENTS (Tables)

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

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

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

	December 31, 2019						December 31, 2018
	Level 1		Level 2		Total		Level 1		Level 2		Total
Assets
Marketable securities
U.S. government and agency securities	$	330	$	—	$	330	$	293	$	—	$	293
Equity mutual funds	6		—		6		—		—		—
Foreign government bonds	—		31		31		—		26		26
Asset-backed securities	—		211		211		—		190		190
Mortgage-backed securities	—		176		176		—		92		92
Corporate notes and bonds	—		523		523		—		520		520
Municipal securities	—		23		23		—		10		10
Total Marketable securities	336		964		1,300		293		838		1,131
Derivative instruments
Fuel hedge contracts - call options	—		11		11		—		4		4
Interest rate swap agreements	—		3		3		—		10		10
Total Assets	$	336	$	978	$	1,314	$	293	$	852	$	1,145
Liabilities
Derivative instruments
Interest rate swap agreements	—		(10)		(10)		—		(7)		(7)
Total Liabilities	$	—	$	(10)	$	(10)	$	—	$	(7)	$	(7)

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

	December 31, 2019		December 31, 2018
Fixed rate debt at cost	$	473	$	639
Non-recurring purchase price accounting fair value adjustment	2		3
Total fixed rate debt	$	475	$	642
Estimated fair value	$	483	$	641

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

Proceeds from sales of marketable securities were $1.7 billion, $1.1 billion and $1.4 billion in 2019, 2018, and 2017.

Schedule of Debt Investment Maturities [Table Text Block]

Maturities for marketable securities (in millions):

December 31, 2019	Cost Basis		Fair Value
Due in one year or less	$	164	$	165
Due after one year through five years	1,102		1,114
Due after five years through 10 years	16		16
Total	$	1,282	$	1,295