AOCI - Differences between Net Income and Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Condensed Statement of Income Captions [Line Items]
Net income (loss)	$ 116	$ (94)
Other, net of deferred taxes	(47)	(28)
Total other comprehensive income (loss)	64	(125)
Comprehensive income (loss)	180	(219)
Other deferred taxes	(13)	(9)
Fuel derivatives
Condensed Statement of Income Captions [Line Items]
Unrealized gain (loss) on derivative instruments, net of deferred taxes	101	(65)
Derivative deferred taxes	30	(19)
Interest rate derivatives
Condensed Statement of Income Captions [Line Items]
Unrealized gain (loss) on derivative instruments, net of deferred taxes	10	(32)
Derivative deferred taxes	$ 3	$ (10)