Fair Value Measurement - Fair Value Assets and Liabilities Measured on Recurring Basis with Unobservable Inputs (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
Total losses (realized or unrealized) included in other comprehensive income
Gains (Losses) included in Other Comprehensive Income (Loss)		$ (112)	$ (1)
Fuel derivatives
Total losses (realized or unrealized) included in other comprehensive income
Beginning Balance		138	248
Purchases	[1]	133	66
Sales	[1]	(2)	(4)
Settlements		(47)	(171)
Ending Balance		110	138
Quoted prices in active markets for identical assets (Level 1) | Fair Value, Measurements, Recurring
Total losses (realized or unrealized) included in other comprehensive income
Interest Rate Derivative Assets, at Fair Value		0
Assets, Fair Value Disclosure		3,392	3,101
Significant other observable inputs (Level 2) | Fair Value, Measurements, Recurring
Total losses (realized or unrealized) included in other comprehensive income
Interest Rate Derivative Assets, at Fair Value		2
Assets, Fair Value Disclosure		879	715
Significant unobservable inputs (Level 3) | Fair Value, Measurements, Recurring
Total losses (realized or unrealized) included in other comprehensive income
Interest Rate Derivative Assets, at Fair Value		0
Assets, Fair Value Disclosure		110	138
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring
Total losses (realized or unrealized) included in other comprehensive income
Interest Rate Derivative Assets, at Fair Value		2
Assets, Fair Value Disclosure		$ 4,381	$ 3,954

[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 was purchased as a single instrument or separate instruments.