Fair Value Measurement - Fair Value Assets and Liabilities Measured on Recurring Basis with Unobservable Inputs (Details) - USD ($) $ in Millions		3 Months Ended	6 Months Ended
		Jun. 30, 2016	Jun. 30, 2016
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance		$ 1,455	$ 1,649
Total gains or (losses) (realized or unrealized)
Included in earnings		(208)	(174)
Included in other comprehensive income		187	134
Purchases		59	130
Sales		(28)	(57)
Settlements		(148)	(387)
June 30, 2016		881	881
The amount of total gains for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held at June 30, 2016		190	170
Fuel derivatives
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance		1,481	1,676
Total gains or (losses) (realized or unrealized)
Included in earnings		(208)	(174)
Included in other comprehensive income		187	135
Purchases	[1]	59	130
Sales	[1]	(28)	(57)
Settlements		(148)	(387)
June 30, 2016		907	907
The amount of total gains for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held at June 30, 2016		190	170
Other securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		26	27
Total gains or (losses) (realized or unrealized)
Included in earnings		0	0
Included in other comprehensive income		0	(1)
Purchases		0	0
Sales		0	0
Settlements		0	0
June 30, 2016		26	26
The amount of total gains for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held at June 30, 2016		$ 0	$ 0

[1] The purchase and sale of fuel derivatives are recorded gross based on the structure of the derivative instrument and whether a contract with multiple derivatives is purchased as a single instrument or separate instruments.