Fair Value Measurement - Fair Value Assets and Liabilities Measured on Recurring Basis with Unobservable Inputs (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value		$ (1,455)	$ (1,649)
Included in earnings		34
Included in other comprehensive income		(53)
Purchases		71
Sales		(29)
Settlements		(239)
The amount of total losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held at March 31, 2016		21
Fuel derivatives
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value		(1,481)	(1,676)
Included in earnings		34
Included in other comprehensive income		(52)
Purchases	[1]	71
Sales	[1]	(29)
Settlements		(239)
The amount of total losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held at March 31, 2016		21
Other securities
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		26	27
Included in earnings		0
Included in other comprehensive income		(1)
Purchases		0
Sales		0
Settlements		0
The amount of total losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held at March 31, 2016		0
Estimate of Fair Value Measurement [Member] | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure		2,875	3,465
Assets, Fair Value Disclosure, Recurring		5,108	5,081
Quoted prices in active markets for identical assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure		0	0
Assets, Fair Value Disclosure, Recurring		3,214	2,651
Significant other observable inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure		645	823
Assets, Fair Value Disclosure, Recurring		1,119	1,437
Significant unobservable inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure		2,230	2,642
Assets, Fair Value Disclosure, Recurring		$ 775	$ 993

[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.