Financial Derivative Instruments - Fair Values by Balance Sheet Location (Details) - USD ($) $ in Millions		Sep. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 1,736	$ 1,360
Derivative Liability, Fair Value, Gross Liability	[1]	3,014	2,409
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	96	13
Derivative Liability, Fair Value, Gross Liability	[1]	1,064	704
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	1,640	1,347
Derivative Liability, Fair Value, Gross Liability	[1]	1,950	1,705
Fuel derivatives
Derivatives, Fair Value [Line Items]
Collateral Already Posted, Aggregate Fair Value		478
Fuel derivatives | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Collateral Already Posted, Aggregate Fair Value		15	68
Fuel derivatives | Accounts Payable Balance Sheet Location [Member]
Derivatives, Fair Value [Line Items]
Due To Third Parties For Settled Fuel Contracts		41	16
Fuel derivatives | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Collateral Already Posted, Aggregate Fair Value		463	198
Fuel derivatives | Designated as Hedging Instrument | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	36	0
Derivative Liability, Fair Value, Gross Liability	[1]	414	0
Fuel derivatives | Designated as Hedging Instrument | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	54	0
Derivative Liability, Fair Value, Gross Liability	[1]	596	643
Fuel derivatives | Not Designated as Hedging Instrument | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	1,358	1,190
Derivative Liability, Fair Value, Gross Liability	[1]	1,536	1,432
Fuel derivatives | Not Designated as Hedging Instrument | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	282	157
Derivative Liability, Fair Value, Gross Liability	[1]	414	273
Interest rate derivatives | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	6	13
Derivative Liability, Fair Value, Gross Liability	[1]	0	0
Interest rate derivatives | Designated as Hedging Instrument | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	0	0
Derivative Liability, Fair Value, Gross Liability	[1]	$ 54	$ 61

[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.