Financial Derivative Instruments - Fair Values by Balance Sheet Location (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 734		$ 1,004
Derivative Liability, Fair Value, Gross Liability	620		880
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	363		386
Derivative Liability, Fair Value, Gross Liability	125		142
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	371		618
Derivative Liability, Fair Value, Gross Liability	495		738
Fuel derivatives | Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Receivable from third parties for settled fuel contracts - noncurrent	54		54
Fuel derivatives | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Cash collateral deposits provided to counterparty - noncurrent	61		89
Fuel derivatives | Prepaid Expenses
Derivatives, Fair Value [Line Items]
Prepaid settlement for fuel contracts - current	0		15
Fuel derivatives | Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	42	[1]	0	[1]
Derivative Liability, Fair Value, Gross Liability	0	[1]	0	[1]
Fuel derivatives | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	292	[1]	355	[1]
Derivative Liability, Fair Value, Gross Liability	10	[1]	16	[1]
Fuel derivatives | Not Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	259	[1]	375	[1]
Derivative Liability, Fair Value, Gross Liability	261	[1]	327	[1]
Fuel derivatives | Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	112	[1]	233	[1]
Derivative Liability, Fair Value, Gross Liability	222	[1]	351	[1]
Fuel derivatives | Not Designated as Hedging Instrument | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	[1]	10	[1]
Derivative Liability, Fair Value, Gross Liability	0	[1]	60	[1]
Fuel derivatives | Not Designated as Hedging Instrument | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	[1]	0	[1]
Derivative Liability, Fair Value, Gross Liability	12	[1]	0	[1]
Interest rate derivatives | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	29	[1]	31	[1]
Derivative Liability, Fair Value, Gross Liability	0	[1]	0	[1]
Interest rate derivatives | Designated as Hedging Instrument | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	[1]	0	[1]
Derivative Liability, Fair Value, Gross Liability	$ 115	[1]	$ 126	[1]

[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.