FAIR VALUE MEASUREMENTS (Tables)

3 Months Ended

Mar. 31, 2021

Fair Value Disclosures [Abstract]

Investments Classified by Contractual Maturity Date

Maturities for marketable securities (in millions):

March 31, 2021	Cost Basis		Fair Value
Due in one year or less	$	1,158	$	1,159
Due after one year through five years	1,231		1,246
Due after five years through 10 years	62		61
Total	$	2,451	$	2,466

Fair Value, by Balance Sheet Grouping

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

	March 31, 2021						December 31, 2020
	Level 1		Level 2		Total		Level 1		Level 2		Total
Assets
Marketable securities
U.S. government and agency securities	$	301	$	—	$	301	$	407	$	—	$	407
Equity mutual funds	7		—		7		7		—		7
Foreign government bonds	—		19		19		—		20		20
Asset-backed securities	—		284		284		—		224		224
Mortgage-backed securities	—		293		293		—		290		290
Corporate notes and bonds	—		1,515		1,515		—		978		978
Municipal securities	—		54		54		—		50		50
Total Marketable securities	308		2,165		2,473		414		1,562		1,976
Derivative instruments
Fuel hedge - call options	—		41		41		—		15		15
Total Assets	$	308	$	2,206	$	2,514	$	414	$	1,577	$	1,991
Liabilities
Derivative instruments
Interest rate swap agreements	—		(18)		(18)		—		(25)		(25)
Total Liabilities	$	—	$	(18)	$	(18)	$	—	$	(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):

	March 31, 2021		December 31, 2020
Total fixed-rate debt	$	1,728	$	1,662
Estimated fair value	$	1,839	$	1,778