Fair Value Measurement - Fair Value Assets and Liabilities Measured on Recurring Basis with Unobservable Inputs (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 1,937	$ 1,360
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		(1,059)	216
Total gains or (losses) (realized or unrealized)
Included in earnings		(647)	(439)
Included in other comprehensive income		(858)	(1,088)
Purchases		750	403
Sales		(200)	(170)
Settlements		365	19
Ending Balance		(1,649)	(1,059)
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 December 31, 2015		(428)	(450)
Fuel derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		(1,091)	172
Total gains or (losses) (realized or unrealized)
Included in earnings		(646)	(439)
Included in other comprehensive income		(858)	(1,091)
Purchases	[2]	750	403
Sales	[2]	(196)	(155)
Settlements		365	19
Ending Balance		(1,676)	(1,091)
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 December 31, 2015		(428)	(450)
Other securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance		32	44
Total gains or (losses) (realized or unrealized)
Included in earnings		(1)	0
Included in other comprehensive income		0	3
Purchases		0	0
Sales		(4)	(15)
Settlements		0	0
Ending Balance		27	32
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 December 31, 2015		0	$ 0
Quoted prices in active markets for identical assets (Level 1) | Swap | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Asset, Fair Value, Gross Asset	[3]	0
Quoted prices in active markets for identical assets (Level 1) | Options Held | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Asset, Fair Value, Gross Asset	[3]	0
Significant other observable inputs (Level 2) | Swap | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Asset, Fair Value, Gross Asset	[3]	38
Significant other observable inputs (Level 2) | Options Held | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Asset, Fair Value, Gross Asset	[3]	0
Significant unobservable inputs (Level 3) | Swap | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Asset, Fair Value, Gross Asset	[3]	0
Significant unobservable inputs (Level 3) | Options Held | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Asset, Fair Value, Gross Asset	[3]	10
Estimate of Fair Value Measurement [Member] | Swap | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Asset, Fair Value, Gross Asset	[3]	38
Estimate of Fair Value Measurement [Member] | Options Held | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Derivative Asset, Fair Value, Gross Asset	[3]	$ 10

[1] Represents the position of each trade before consideration of offsetting positions with each counterparty and does not include the impact of cash collateral deposits provided to or received from counterparties. See discussion of credit risk and collateral following in this Note.

[2] 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.

[3] In the Consolidated Balance Sheet amounts are presented as a net asset. See Note 10.