Fair Value Measurement - Fair Value Assets and Liabilities Measured on Recurring Basis with Unobservable Inputs (Details) - Fuel derivatives $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($)
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 258
Total gains or (losses) (realized or unrealized)
Included in earnings	89
Included in other comprehensive income	(125)
Purchases	33	[1]
Sales	0	[1]
Settlements	(135)
March 31, 2017	304
The amount of total gains for the period included in earnings attributable to the change in unrealized gains or losses relating to option contracts still held at March 31, 2017	$ (77)

[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.