Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Jun. 30, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income Loss [Line Items]
Net income	$ 584	$ 329	$ 1,645	$ 946
Other, net of deferred taxes	(3)	1	(4)	4
Total other comprehensive income (loss)	(239)	(203)	(165)	(89)
Total comprehensive income	345	126	1,480	857
Deferred taxes on unrealized gain (loss) on other	(2)	1	(1)	4
AOCI Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(903)		(903)		$ (664)	$ (738)
Reclassification to earnings	82		175
Changes in fair value	(321)		(340)
Fuel derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(1,438)		(1,438)		(1,063)	(1,177)
Reclassification to earnings	126		264
Changes in fair value	(501)		(525)
Interest rate derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(40)		(40)		(40)	(45)
Reclassification to earnings	5		15
Changes in fair value	(5)		(10)
Defined benefit plan items
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	41		41		41	41
Reclassification to earnings	0		0
Changes in fair value	0		0
Other
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	3		3		8	8
Reclassification to earnings	0		0
Changes in fair value	(5)		(5)
Deferred tax
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	531		531		$ 390	$ 435
Reclassification to earnings	(49)		(104)
Changes in fair value	190		200
Interest rate derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	0	6	3	8
Deferred taxes on unrealized gain (loss) on derivatives	0	3	2	4
Fuel derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	(236)	(210)	(164)	(101)
Deferred taxes on unrealized gain (loss) on derivatives	$ (139)	$ (124)	$ (97)	$ (60)