Fair Value Measurement - Fair Value Assets and Liabilities Measured on Recurring Basis with Unobservable Inputs (Details) - Fuel derivatives - USD ($) $ in Millions		3 Months Ended	6 Months Ended
		Jun. 30, 2017	Jun. 30, 2017
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance		$ (304)	$ (258)
Total gains or (losses) (realized or unrealized)
Included in earnings		(57)	(146)
Included in other comprehensive income		(84)	(209)
Purchases	[1]	46	79
Sales	[1]	0	0
Settlements		(166)	(301)
June 30, 2017		233	233
The amount of total losses for the period included in earnings attributable to the change in unrealized gains or losses relating to option contracts still held at June 30, 2017		$ (46)	$ (101)

[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.