Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income Loss [Line Items]
Net income	$ 746	$ 820	$ 1,097	$ 1,333
Other, net of deferred taxes	2	0	2	0
Total other comprehensive income (loss)	49	266	60	423
Total comprehensive income	795	1,086	1,157	1,756
Deferred taxes on unrealized gain (loss) on other	0	0	0	0
Interest Expense	(27)	(32)	(56)	(62)
Fuel and oil	(990)	(903)	(1,912)	(1,755)
AOCI Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(263)		(263)		$ (312)	$ (323)
Reclassification to earnings	102		192
Changes in fair value	(53)		(132)
Fuel derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(412)		(412)		(485)	(499)
Changes in fair value	(85)		(210)
Interest rate derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(13)		(13)		(15)	(18)
Changes in fair value	(1)		(1)
Defined benefit plan items
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(14)		(14)		(14)	(14)
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	154		154		182	188
Reclassification to earnings	(59)		(111)
Changes in fair value	31		77
Other
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	22		22		$ 20	$ 20
Reclassification to earnings	0		0
Changes in fair value	2		2
Interest rate derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	1	1	3	(1)
Deferred taxes on unrealized gain (loss) on derivatives	1	1	2	(1)
Fuel derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	46	265	55	424
Deferred taxes on unrealized gain (loss) on derivatives	$ 27	$ 155	$ 32	$ 249