FAIR VALUE MEASUREMENTS (Tables)

12 Months Ended

Dec. 31, 2020

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, 2020						December 31, 2019
	Level 1		Level 2		Total		Level 1		Level 2		Total
Assets
Marketable securities
U.S. government and agency securities	$	407	$	—	$	407	$	330	$	—	$	330
Equity mutual funds	7		—		7		6		—		6
Foreign government bonds	—		20		20		—		31		31
Asset-backed securities	—		224		224		—		211		211
Mortgage-backed securities	—		290		290		—		176		176
Corporate notes and bonds	—		978		978		—		523		523
Municipal securities	—		50		50		—		23		23
Total Marketable securities	414		1,562		1,976		336		964		1,300
Derivative instruments
Fuel hedge contracts - call options	—		15		15		—		11		11
Interest rate swap agreements	—		—		—		—		3		3
Total Assets	$	414	$	1,577	$	1,991	$	336	$	978	$	1,314
Liabilities
Derivative instruments
Interest rate swap agreements	—		(25)		(25)		—		(10)		(10)
Total Liabilities	$	—	$	(25)	$	(25)	$	—	$	(10)	$	(10)

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

	December 31, 2020		December 31, 2019
Total fixed rate debt	$	1,662	$	475
Estimated fair value	$	1,778	$	483

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

Proceeds from sales of marketable securities were $2.3 billion, $1.7 billion and $1.1 billion in 2020, 2019, and 2018.

Schedule of Debt Investment Maturities [Table Text Block]

Maturities for marketable securities (in millions):

December 31, 2020	Cost Basis		Fair Value
Due in one year or less	$	775	$	777
Due after one year through five years	1,146		1,175
Due after five years through 10 years	23		24
Total	$	1,944	$	1,976