Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income Loss [Line Items]
Net income	$ 503	$ 388	$ 1,600	$ 1,722
Other, net of deferred taxes	2	3	4	3
Total other comprehensive income (loss)	127	129	187	552
Total comprehensive income	630	517	1,787	2,274
Deferred taxes on unrealized gain (loss) on other	2	2	2	2
Less: Tax Expense	288	230	913	1,016
Interest Expense	(28)	(31)	(84)	(93)
Fuel derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(412)		(499)
Changes in fair value	45		(165)
Reclassification to earnings	151		448
Ending Balance	(216)		(216)
Interest rate derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(13)		(18)
Changes in fair value	0		(1)
Reclassification to earnings	2		8
Ending Balance	(11)		(11)
Defined benefit plan items
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(14)		(14)
Changes in fair value	0		0
Reclassification to earnings	0		0
Ending Balance	(14)		(14)
Other
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	22		20
Changes in fair value	4		6
Reclassification to earnings	0		0
Ending Balance	26		26
Deferred Tax
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	154		188
Changes in fair value	(18)		59
Reclassification to earnings	(57)		(168)
Ending Balance	79		79
AOCI Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(263)		(323)
Changes in fair value	31		(101)
Reclassification to earnings	96		288
Ending Balance	(136)		(136)
Interest rate derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	2	4	5	3
Deferred taxes on unrealized gain (loss) on derivatives	0	2	2	1
Fuel derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	123	122	178	546
Deferred taxes on unrealized gain (loss) on derivatives	$ 73	$ 72	$ 105	$ 321