Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Accumulated Other Comprehensive Income Loss [Line Items]
Net income	$ 152	$ 59
Other, net of deferred taxes	2	3
Total other comprehensive income (loss)	(9)	8
Total comprehensive income	143	67
Deferred taxes on unrealized gain (loss) on other	1	2
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
AOCI Beginning Balance	(3)
AOCI Ending Balance	(12)
Fuel derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	(13)	(3)
Deferred taxes on unrealized gain (loss) on derivatives	(7)	(1)
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
Reclassification to earnings	0
AOCI Beginning Balance	(20)
Changes in fair value	(20)
AOCI Ending Balance	(40)
Interest rate derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	2	8
Deferred taxes on unrealized gain (loss) on derivatives	1	5
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
Reclassification to earnings	6
AOCI Beginning Balance	(58)
Changes in fair value	(3)
AOCI Ending Balance	(55)
Defined benefit plan items
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
Reclassification to earnings	0
AOCI Beginning Balance	65
Changes in fair value	0
AOCI Ending Balance	65
Other
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
Reclassification to earnings	0
AOCI Beginning Balance	8
Changes in fair value	3
AOCI Ending Balance	11
Deferred tax
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
Reclassification to earnings	(2)
AOCI Beginning Balance	2
Changes in fair value	7
AOCI Ending Balance	7
Accumulated other comprehensive income (loss)
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
Reclassification to earnings	4
AOCI Beginning Balance	(3)
Changes in fair value	(13)
AOCI Ending Balance	$ (12)