Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Accumulated Other Comprehensive Income Loss [Line Items]
Net income	$ 59	$ 98
Other, net of deferred taxes	3	2
Total other comprehensive income (loss)	8	167
Total comprehensive income	67	265
Deferred taxes on unrealized gain (loss) on other	2	2
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
AOCI Beginning Balance	(119)
AOCI Ending Balance	(111)
Fuel derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	(3)	159
Deferred taxes on unrealized gain (loss) on derivatives	(1)	99
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
AOCI Beginning Balance	(63)
Changes in fair value, derivatives	(29)
Reclassification to earnings, derivatives	26
AOCI Ending Balance	(66)
Interest rate derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	8	6
Deferred taxes on unrealized gain (loss) on derivatives	5	4
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
AOCI Beginning Balance	(67)
Changes in fair value, derivatives	3
Reclassification to earnings, derivatives	5
AOCI Ending Balance	(59)
Defined benefit pension items
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
AOCI Beginning Balance	16
Changes in fair value, derivatives	0
Reclassification to earnings, derivatives	0
AOCI Ending Balance	16
Other
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
AOCI Beginning Balance	(5)
Changes in fair value, other	3
Reclassification to earnings, derivatives	0
AOCI Ending Balance	(2)
Accumulated other comprehensive income (loss)
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
AOCI Beginning Balance	(119)
Changes in fair value, other	(23)
Reclassification to earnings, other	31
AOCI Ending Balance	$ (111)