FAIR VALUE MEASUREMENTS (Tables)

6 Months Ended

Jun. 30, 2021

Fair Value Disclosures [Abstract]

Investments Classified by Contractual Maturity Date

Maturities for marketable securities (in millions):

June 30, 2021	Cost Basis		Fair Value
Due in one year or less	$	1,538	$	1,539
Due after one year through five years	1,280		1,294
Due after five years through 10 years	88		88
Total	$	2,906	$	2,921

Fair Value, by Balance Sheet Grouping

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

	June 30, 2021						December 31, 2020
	Level 1		Level 2		Total		Level 1		Level 2		Total
Assets
Marketable securities
U.S. government and agency securities	$	298	$	—	$	298	$	407	$	—	$	407
Equity mutual funds	5		—		5		7		—		7
Foreign government bonds	—		31		31		—		20		20
Asset-backed securities	—		330		330		—		224		224
Mortgage-backed securities	—		253		253		—		290		290
Corporate notes and bonds	—		1,943		1,943		—		978		978
Municipal securities	—		66		66		—		50		50
Total Marketable securities	303		2,623		2,926		414		1,562		1,976
Derivative instruments
Fuel hedge - call options	—		92		92		—		15		15
Total Assets	$	303	$	2,715	$	3,018	$	414	$	1,577	$	1,991
Liabilities
Derivative instruments
Interest rate swap agreements	—		(16)		(16)		—		(25)		(25)
Total Liabilities	$	—	$	(16)	$	(16)	$	—	$	(25)	$	(25)

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

	June 30, 2021		December 31, 2020
Total fixed-rate debt	$	1,896	$	1,662
Estimated fair value	$	2,019	$	1,778