Accumulated Other Comprehensive Income - Schedule of AOCI Components (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Fuel derivatives
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		$ 83	$ 3
Changes in Fair Value		102
Reclassification to earnings		(4)
ASU 2017-12 Adoption Adjustment	[1]	26
Fuel derivatives | Accounting Standards Update 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification to earnings	[2]	0
Interest rate derivatives
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		(4)	(7)
Changes in Fair Value		1
Reclassification to earnings		(2)
ASU 2017-12 Adoption Adjustment	[1]	0
Interest rate derivatives | Accounting Standards Update 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification to earnings	[2]	0
Defined Benefit Plan Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		(9)	(9)
Changes in Fair Value		0
Reclassification to earnings		0
ASU 2017-12 Adoption Adjustment	[1]	0
Defined Benefit Plan Items | Accounting Standards Update 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification to earnings	[2]	0
Other comprehensive income other changes net of tax
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		33	33
Changes in Fair Value		0
Reclassification to earnings		0
ASU 2017-12 Adoption Adjustment	[1]	0
Other comprehensive income other changes net of tax | Accounting Standards Update 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification to earnings	[2]	0
Deferred Tax
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		(25)	(8)
Changes in Fair Value		(24)
Reclassification to earnings		(1)
ASU 2017-12 Adoption Adjustment	[1]	6
Deferred Tax | Accounting Standards Update 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification to earnings	[2]	2
AOCI Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest		78	$ 12
Changes in Fair Value		79
Reclassification to earnings		5
ASU 2017-12 Adoption Adjustment	[1]	(20)
AOCI Including Portion Attributable to Noncontrolling Interest | Accounting Standards Update 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification to earnings	[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. See Note 2 for further information on this adoption.