Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Accumulated Other Comprehensive Income Loss [Line Items]
Net income	$ 351	$ 513
Total other comprehensive income (loss)	11	157
Total comprehensive income	362	670
Deferred taxes on unrealized gain (loss) on other	0	0
Interest Expense	29	30
Fuel and oil	922	852
AOCI Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(312)		$ (323)
Reclassification to earnings	90
Changes in fair value	(79)
Fuel derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(485)		(499)
Changes in fair value	(125)
Interest rate derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(15)		(18)
Changes in fair value	0
Defined benefit plan items
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(14)		(14)
Reclassification to earnings	0
Changes in fair value	0
Deferred Tax
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	182		188
Reclassification to earnings	(52)
Changes in fair value	46
Other
Accumulated Other Comprehensive Income Loss [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	20		$ 20
Reclassification to earnings	0
Changes in fair value	0
Interest rate derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	2	(3)
Deferred taxes on unrealized gain (loss) on derivatives	1	(1)
Fuel derivatives
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on derivatives, net of tax	9	160
Deferred taxes on unrealized gain (loss) on derivatives	$ 5	$ 93