Accumulated Other Comprehensive (Income) Loss (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Fuel and oil		$ 4,347	$ 4,616	$ 4,076
Interest Expense		118	131	114
Fuel derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(56)	3
Changes in fair value		(117)	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax			(33)
Beginning balance		56	(3)
Changes in fair value		(117)	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax			(33)
Ending balance		(125)	(56)	3
Interest rate derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		0	(7)
Changes in fair value		(38)	1
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax			6
Beginning balance		0	7
Changes in fair value		(38)	1
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax			6
Ending balance		(33)	0	(7)
Defined benefit plan items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		58	(9)
Changes in fair value		(38)	67
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		0	0
Beginning balance		(58)	9
Changes in fair value		(38)	67
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		0	0
Ending balance		20	58	(9)
Other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		25	33
Changes in fair value		34	(8)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		0	0
Beginning balance		(25)	(33)
Changes in fair value		34	(8)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		0	0
Ending balance		59	25	33
Deferred tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(7)	(8)
Changes in fair value		37	(14)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(12)	7
Beginning balance		7	8
Changes in fair value		37	(14)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(12)	7
Ending balance		18	(7)	(8)
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		20	12
Changes in fair value		(122)	46
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		41	(20)
Beginning balance		(20)	(12)
Changes in fair value		(122)	46
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		41	(20)
Ending balance		(61)	20	$ 12
Fuel derivatives | Reclassification out of Accumulated Other Comprehensive Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Fuel and oil		48
Interest rate derivatives | Reclassification out of Accumulated Other Comprehensive Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Interest Expense		$ 5
ASU 2017-12 | Fuel derivatives
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	[1]		(26)
ASU 2017-12 | Reclassificiation out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	[1]		6
ASU 2017-12 | Interest rate derivatives
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	[1]		0
ASU 2017-12 | Defined benefit plan items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	[1]		0
ASU 2017-12 | Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	[1]		0
ASU 2017-12 | Accumulated other comprehensive income (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	[1]		(20)
Accounting Standards Update 2018-02 [Member] | Fuel derivatives
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	[2]		0
Accounting Standards Update 2018-02 [Member] | Reclassificiation out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	[2]		2
Accounting Standards Update 2018-02 [Member] | Interest rate derivatives
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	[2]		0
Accounting Standards Update 2018-02 [Member] | Defined benefit plan items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	[2]		0
Accounting Standards Update 2018-02 [Member] | Other
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	[2]		0
Accounting Standards Update 2018-02 [Member] | Accumulated other comprehensive income (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	[2]		$ 2

[1] The Company adopted the New Hedging Standard as of January 1, 2018. See Note 2 for further information on this adoption.

[2] The Company adopted the Reclassification of Certain Tax Effects from AOCI as of January 1, 2018, which allowed the Company to reclassify to Retained earnings any tax effects stranded in AOCI as a result of the Tax Cuts and Jobs Act enacted in December 2017.