Comprehensive Income (Loss) - Differences between Net Income and Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Condensed Statement of Income Captions [Line Items]
NET INCOME (LOSS)	$ 348		$ (915)		$ 463	$ (1,009)
Other, net of deferred taxes of $— and $8	0		25		(47)	(3)
Total other comprehensive income (loss)	196		56		260	(69)
COMPREHENSIVE INCOME (LOSS)	544	$ 180	(859)	$ (219)	723	(1,078)
Other deferred taxes	0		8		(13)	(1)
Fuel derivatives
Condensed Statement of Income Captions [Line Items]
Unrealized gain (loss) on derivative instruments, net of deferred taxes	201		30		301	(35)
Derivative deferred taxes	61		9		92	(10)
Interest rate derivatives
Condensed Statement of Income Captions [Line Items]
Unrealized gain (loss) on derivative instruments, net of deferred taxes	(5)		1		6	(31)
Derivative deferred taxes	$ (1)		$ 0		$ 1	$ (10)