FAIR VALUE MEASUREMENTS (Tables)

3 Months Ended

Mar. 31, 2019

Fair Value Disclosures [Abstract]

Activity for Marketable Securities

Investments Classified by Contractual Maturity Date

Maturities for marketable securities (in millions):

March 31, 2019	Cost Basis		Fair Value
Due in one year or less	$	217	$	217
Due after one year through five years	991		993
Due after five years through 10 years	11		11
Total	$	1,219	$	1,221

Fair Value, by Balance Sheet Grouping

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

	March 31, 2019								December 31, 2018
	Level 1		Level 2			Total			Level 1		Level 2			Total
Assets
Marketable securities
U.S. government and agency securities	$	367	$	—		$	367		$	293	$	—		$	293
Foreign government bonds	—		22			22			—		26			26
Asset-backed securities	—		217			217			—		190			190
Mortgage-backed securities	—		87			87			—		92			92
Corporate notes and bonds	—		514			514			—		520			520
Municipal securities	—		14			14			—		10			10
Total Marketable securities	367		854			1,221			293		838			1,131
Derivative instruments
Fuel hedge—call options	—		9			9			—		4			4
Interest rate swap agreements	—		8			8			—		10			10
Total Assets	$	367	$	871		$	1,238		$	293	$	852		$	1,145
Liabilities
Derivative instruments
Interest rate swap agreements	—		(8		)	(8		)	—		(7		)	(7		)
Total Liabilities	$	—	$	(8	)	$	(8	)	$	—	$	(7	)	$	(7	)

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

	March 31, 2019		December 31, 2018
Fixed-rate debt at cost	$	512	$	639
Non-recurring purchase price accounting fair value adjustment	3		3
Total fixed-rate debt	$	515	$	642
Estimated fair value	$	518	$	641