Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended	6 Months Ended		3 Months Ended	6 Months Ended		3 Months Ended	6 Months Ended		3 Months Ended	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 Fuel derivatives	Jun. 30, 2013 Fuel derivatives	Jun. 30, 2014 Fuel derivatives	Jun. 30, 2013 Fuel derivatives	Jun. 30, 2014 Interest rate derivatives	Jun. 30, 2013 Interest rate derivatives	Jun. 30, 2014 Interest rate derivatives	Jun. 30, 2013 Interest rate derivatives	Jun. 30, 2014 Defined benefit plan items	Jun. 30, 2014 Defined benefit plan items	Mar. 31, 2013 Defined benefit plan items	Jun. 30, 2014 Other	Jun. 30, 2014 Other	Mar. 31, 2013 Other	Jun. 30, 2014 Deferred tax	Jun. 30, 2014 Deferred tax	Mar. 31, 2013 Deferred tax	Jun. 30, 2014 Accumulated other comprehensive income (loss)	Jun. 30, 2014 Accumulated other comprehensive income (loss)	Mar. 31, 2013 Accumulated other comprehensive income (loss)
Accumulated Other Comprehensive Income Loss [Line Items]
Net income	$ 465	$ 224	$ 617	$ 283
Unrealized gain (loss) on derivatives, net of tax					121	(152)	109	(155)	0	15	2	23
Other, net of deferred taxes	2	0	3	3
Total other comprehensive income (loss)	123	(137)	114	(129)
Total comprehensive income	588	87	731	154
Deferred taxes on unrealized gain (loss) on derivatives					72	(90)	64	(91)	0	9	1	15
Deferred taxes on unrealized gain (loss) on other	1	0	3	2
Accumulated Other Comprehensive Income Loss Net Of Tax [Roll Forward]
Reclassification to earnings					(2)		(2)		5		11		0	0		0	0		(1)	(3)		2	6
AOCI Beginning Balance			(3)			(40)	(20)			(55)	(58)			65	65		8	11		2	7		(3)	(12)
Changes in fair value					195		175		(5)		(8)		0	0		3	6		(72)	(65)		121	108
AOCI Ending Balance	$ 111		$ 111		$ 153		$ 153		$ (55)		$ (55)		$ 65	$ 65	$ 65	$ 14	$ 14	$ 11	$ (66)	$ (66)	$ 7	$ 111	$ 111	$ (12)