AOCI - Differences between Net Income and Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Condensed Statement of Income Captions [Line Items]
NET INCOME	$ 741		$ 733		$ 1,128	$ 1,195
Other, net of deferred taxes	5		3		16	3
Total other comprehensive income (loss)	(66)		220		10	304
Comprehensive income	675	$ 463	953	$ 547	1,138	1,499
Other deferred taxes	1		0		4	0
Fuel derivatives
Condensed Statement of Income Captions [Line Items]
Unrealized gain (loss) on derivative instruments, net of deferred taxes	(56)		216		19	297
Derivative deferred taxes	(17)		66		6	91
Interest rate derivatives
Condensed Statement of Income Captions [Line Items]
Unrealized gain (loss) on derivative instruments, net of deferred taxes	(15)		1		(25)	4
Derivative deferred taxes	$ (4)		$ 1		$ (8)	$ 1