Fair Value Measurement - Fair Value Assets and Liabilities Measured on Recurring Basis with Unobservable Inputs (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Gain (Loss) Included in Other Comprehensive Income (Loss)	$ 101
Fuel derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at December 31, 2017	248
Purchases	18	[1]
Settlements	(27)
Balance at March 31, 2018	$ 340

[1] The purchase 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.