Comprehensive Income (Loss) - Schedule of AOCI Components (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2021
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 9,093	$ 8,876	$ 8,876
Changes in fair value	186		279
Reclassification to earnings	10		(19)
Balance at end of period	9,688	$ 9,093	$ 9,688
Accounting Standards Update [Extensible Enumeration]		Accounting Standards Update 2016-01	Accounting Standards Update 2016-01
Cumulative effect of adopting Accounting Standards Update No. 2016-01, Financial Instruments (See Note 1)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		$ 0	$ 0
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(60)	(105)	(105)
Balance at end of period	136	(60)	136
Accumulated other comprehensive income (loss) | Cumulative effect of adopting Accounting Standards Update No. 2016-01, Financial Instruments (See Note 1)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		(19)	(19)
Fuel and Interest Rate Derivatives | Fuel derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	12	(119)	(119)
Changes in fair value	250		359
Reclassification to earnings	12		34
Balance at end of period	274	12	274
Fuel and Interest Rate Derivatives | Interest rate derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(53)	(66)	(66)
Changes in fair value	(7)		5
Reclassification to earnings	1		2
Balance at end of period	(59)	(53)	(59)
Defined benefit plan items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(43)	(43)	(43)
Balance at end of period	(43)	(43)	(43)
Other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		91	91
Reclassification to earnings			(60)
Balance at end of period	0		0
Other | Cumulative effect of adopting Accounting Standards Update No. 2016-01, Financial Instruments (See Note 1)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		(31)	(31)
Deferred tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	24	32	32
Changes in fair value	(57)		(85)
Reclassification to earnings	(3)		5
Balance at end of period	$ (36)	24	(36)
Deferred tax | Cumulative effect of adopting Accounting Standards Update No. 2016-01, Financial Instruments (See Note 1)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		$ 12	$ 12