FAIR VALUE MEASUREMENTS (Tables)

3 Months Ended

Mar. 31, 2018

Fair Value Disclosures [Abstract]

Activity for Marketable Securities

Investments Classified by Contractual Maturity Date

Maturities for marketable securities (in millions):

March 31, 2018	Cost Basis		Fair Value
Due in one year or less	$	141	$	141
Due after one year through five years	1,207		1,187
Due after five years through 10 years	23		23
Total	$	1,371	$	1,351

Fair Value, by Balance Sheet Grouping

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

March 31, 2018	Level 1		Level 2			Total
Assets
Marketable securities
U.S. government and agency securities	$	330	$	—		$	330
Foreign government bonds	—		32			32
Asset-backed securities	—		208			208
Mortgage-backed securities	—		86			86
Corporate notes and bonds	—		685			685
Municipal securities	—		10			10
Total Marketable securities	330		1,021			1,351
Derivative instruments
Fuel hedge—call options	—		35			35
Interest rate swap agreements	—		13			13
Total Assets	330		1,069			1,399
Liabilities
Derivative instruments
Interest rate swap agreements	—		(6		)	(6		)
Total Liabilities	—		(6		)	(6		)

December 31, 2017	Level 1		Level 2			Total
Assets
Marketable securities
U.S. government and agency securities	$	328	$	—		$	328
Foreign government bonds	—		43			43
Asset-backed securities	—		209			209
Mortgage-backed securities	—		99			99
Corporate notes and bonds	—		726			726
Municipal securities	—		22			22
Total Marketable securities	328		1,099			1,427
Derivative instruments
Fuel hedge—call options	—		22			22
Interest rate swap agreements	—		9			9
Total Assets	328		1,130			1,458
Liabilities
Derivative instruments
Interest rate swap agreements	—		(8		)	(8		)
Total Liabilities	—		(8		)	(8		)

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, 2018		December 31, 2017
Fixed-rate debt at cost	$	874	$	956
Non-recurring purchase price accounting fair value adjustment	3		3
Total fixed-rate debt	877		959
Estimated fair value	871		959