Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Accumulated Other Comprehensive Income Loss [Line Items]
Net income	$ 224	$ 228	$ 283	$ 327
Other, net of deferred taxes	0	(4)	3	(2)
Total other comprehensive income (loss)	(137)	(263)	(129)	(96)
Total comprehensive income	87	(35)	154	231
Deferred taxes on unrealized gain (loss) on other	0	(3)	2	(1)
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
AOCI Beginning Balance			(119)
AOCI Ending Balance	(248)		(248)
Fuel derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	(152)	(251)	(155)	(92)
Deferred taxes on unrealized gain (loss) on derivatives	(90)	(156)	(91)	(57)
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
AOCI Beginning Balance	(107)		(103)
Changes in fair value, derivatives	(301)		(347)
Reclassification to earnings, derivatives	59		101
Deferred tax			(91)
AOCI Ending Balance	(349)		(349)
Interest rate derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	15	(8)	23	(2)
Deferred taxes on unrealized gain (loss) on derivatives	9	(5)	15	(1)
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
AOCI Beginning Balance	(94)		(108)
Changes in fair value, derivatives	18		24
Reclassification to earnings, derivatives	6		14
Deferred tax			15
AOCI Ending Balance	(70)		(70)
Defined benefit pension items
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
AOCI Beginning Balance	26		26
Changes in fair value, derivatives	0		0
Reclassification to earnings, derivatives	0		0
Deferred tax			0
AOCI Ending Balance	26		26
Other
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
AOCI Beginning Balance	(3)		(8)
Changes in fair value, other	0		5
Reclassification to earnings, derivatives	0		0
Deferred tax			2
AOCI Ending Balance	(3)		(3)
Deferred tax
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
AOCI Beginning Balance	67		74
Changes in fair value, other	105		117
Reclassification to earnings, derivatives	(24)		(43)
Deferred tax			74
AOCI Ending Balance	148		148
Accumulated other comprehensive income (loss)
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
AOCI Beginning Balance	(111)		(119)
Changes in fair value, other	(178)		(201)
Reclassification to earnings, other	41		72
Deferred tax			0
AOCI Ending Balance	$ (248)		$ (248)