AOCI - Differences between Net Income and Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Condensed Statement of Income Captions [Line Items]
NET INCOME	$ 387	$ 463
Other, net of deferred taxes	11	0
Total other comprehensive income	76	84
Comprehensive income	463	547
Other deferred taxes	3	0
Fuel derivatives
Condensed Statement of Income Captions [Line Items]
Unrealized gain (loss) on derivative instruments, net of deferred taxes	76	81
Derivative deferred taxes	22	25
Interest rate derivatives
Condensed Statement of Income Captions [Line Items]
Unrealized gain (loss) on derivative instruments, net of deferred taxes	(11)	3
Derivative deferred taxes	$ (3)	$ 0