Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Mar. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income Loss [Line Items]
Net income	$ 820	$ 608	$ 1,333	$ 1,061
Total other comprehensive income (loss)	266	78	423	74
Total comprehensive income	1,086	686	1,756	1,135
AOCI Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(628)		(628)		$ (894)	$ (1,051)
Reclassification to earnings	152		349
Changes in fair value	114		74
Fuel derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(993)		(993)		(1,413)	(1,666)
Reclassification to earnings	236		546
Changes in fair value	184		127
Interest rate derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(32)		(32)		(34)	(30)
Reclassification to earnings	4		8
Changes in fair value	(2)		(10)
Defined benefit plan items
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	22		22		22	22
Reclassification to earnings	0		0
Changes in fair value	0		0
Other
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	6		6		6	6
Reclassification to earnings	0		0
Changes in fair value	0		0
Deferred tax
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	369		369		$ 525	$ 617
Reclassification to earnings	(88)		(205)
Changes in fair value	(68)		(43)
Interest rate derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	1	4	(1)	3
Deferred taxes on unrealized gain (loss) on derivatives	1	2	(1)	2
Fuel derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	265	74	424	71
Deferred taxes on unrealized gain (loss) on derivatives	$ 155	$ 44	$ 249	$ 43