Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Accumulated Other Comprehensive Income Loss [Line Items]
Net income	$ 608	$ 465	$ 1,061	$ 617
Other, net of deferred taxes	0	2	0	3
Total other comprehensive income (loss)	78	123	74	114
Total comprehensive income	686	588	1,135	731
Deferred taxes on unrealized gain (loss) on other	0	1	0	33
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
AOCI Beginning Balance			(738)
AOCI Ending Balance	(664)		(664)
Fuel derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	74	121	71	109
Deferred taxes on unrealized gain (loss) on derivatives	44	72	43	64
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
Reclassification to earnings	65		138
AOCI Beginning Balance	(1,181)		(1,177)
Changes in fair value	53		(24)
AOCI Ending Balance	(1,063)		(1,063)
Interest rate derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	4	0	3	2
Deferred taxes on unrealized gain (loss) on derivatives	2	$ 0	2	$ 1
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
Reclassification to earnings	5		10
AOCI Beginning Balance	(46)		(45)
Changes in fair value	1		(5)
AOCI Ending Balance	(40)		(40)
Defined benefit plan items
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
Reclassification to earnings	0		0
AOCI Beginning Balance	41		41
Changes in fair value	0		0
AOCI Ending Balance	41		41
Other
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
Reclassification to earnings	0		0
AOCI Beginning Balance	8		8
Changes in fair value	0		0
AOCI Ending Balance	8		8
Deferred tax
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
Reclassification to earnings	(26)		(55)
AOCI Beginning Balance	436		435
Changes in fair value	(20)		10
AOCI Ending Balance	390		390
Accumulated other comprehensive income (loss)
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
Reclassification to earnings	44		93
AOCI Beginning Balance	(742)		(738)
Changes in fair value	34		(19)
AOCI Ending Balance	$ (664)		$ (664)