Financial Derivative Instruments - Fair Values by Balance Sheet Location (Details) - USD ($) $ in Millions		Mar. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 1,437	$ 1,937
Derivative Liability, Fair Value, Gross Liability	[1]	2,875	3,465
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	151	168
Derivative Liability, Fair Value, Gross Liability	[1]	1,118	1,233
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	1,286	1,769
Derivative Liability, Fair Value, Gross Liability	[1]	1,757	2,232
Fuel derivatives
Derivatives, Fair Value [Line Items]
Collateral Already Posted, Aggregate Fair Value		1,067
Fuel derivatives | Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Total Collateral Already Received Aggregate Fair Value		9	0
Fuel derivatives | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Collateral Already Posted, Aggregate Fair Value		586	235
Fuel derivatives | Accounts Payable
Derivatives, Fair Value [Line Items]
Due To Third Parties For Settled Fuel Contracts		53	46
Fuel derivatives | Other Assets
Derivatives, Fair Value [Line Items]
Total Collateral Already Received Aggregate Fair Value		4	0
Fuel derivatives | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Collateral Already Posted, Aggregate Fair Value		494	600
Fuel derivatives | Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	1	2
Derivative Asset, Fair Value, Gross Liability	[1]	0	0
Fuel derivatives | Designated as Hedging Instrument | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	[1]	74	107
Derivative Liability, Fair Value, Gross Liability	[1]	548	526
Fuel derivatives | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	6	2
Derivative Asset, Fair Value, Gross Liability	[1]	26	0
Fuel derivatives | Designated as Hedging Instrument | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	[1]	54	55
Derivative Liability, Fair Value, Gross Liability	[1]	498	658
Fuel derivatives | Not Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	29	39
Derivative Asset, Fair Value, Gross Liability	[1]	19	26
Fuel derivatives | Not Designated as Hedging Instrument | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	[1]	920	1,395
Derivative Liability, Fair Value, Gross Liability	[1]	1,390	1,854
Fuel derivatives | Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	39	5
Derivative Asset, Fair Value, Gross Liability	[1]	0	0
Fuel derivatives | Not Designated as Hedging Instrument | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	[1]	298	330
Derivative Liability, Fair Value, Gross Liability	[1]	348	352
Interest rate derivatives | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	16	2
Derivative Asset, Fair Value, Gross Liability	[1]	0	0
Interest rate derivatives | Designated as Hedging Instrument | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	[1]	0	0
Derivative Liability, Fair Value, Gross Liability	[1]	$ 46	$ 49

[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.