Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fuel derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ (125)	$ (56)
Changes in fair value	(103)	(117)
Ending balance	(119)	(125)
Interest rate derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(33)	0
Changes in fair value	(34)	(38)
Reclassification to earnings	1	5
Ending balance	(66)	(33)
Defined benefit plan items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	20	58
Changes in fair value	(63)	(38)
Reclassification to earnings	0	0
Ending balance	(43)	20
Other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	59	25
Changes in fair value	32	34
Reclassification to earnings	0	0
Ending balance	91	59
Deferred tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	18	(7)
Changes in fair value	39	37
Reclassification to earnings	(25)	(12)
Ending balance	32	18
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(61)	20
Changes in fair value	(129)	(122)
Reclassification to earnings	85	41
Ending balance	(105)	(61)
Reclassification out of Accumulated Other Comprehensive Income | Fuel derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification to earnings	$ 109	$ 48