Financial Derivative Instruments - Fair Values by Balance Sheet Location (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 436	[1]	$ 1,004	[1]
Derivative Liability, Fair Value, Gross Liability	596	[1]	880	[1]
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	192	[1]	386	[1]
Derivative Liability, Fair Value, Gross Liability	205	[1]	142	[1]
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	244	[1]	618	[1]
Derivative Liability, Fair Value, Gross Liability	391	[1]	738	[1]
Fuel derivatives | Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Prepaid settlement for fuel contracts - current	0		15
Fuel derivatives | Other Assets
Derivatives, Fair Value [Line Items]
Receivable from third parties for settled fuel contracts - noncurrent	27		54
Fuel derivatives | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Cash collateral deposits provided to counterparty - current	55		0
Due To Third Parties For Settled Fuel Contracts	8		0
Fuel derivatives | Accounts And Other Receivables [Member]
Derivatives, Fair Value [Line Items]
Receivable from third parties for settled fuel contracts - current	35		0
Fuel derivatives | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Cash collateral deposits provided to counterparty - noncurrent	10		0
Fuel derivatives | Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	17	[1]	0	[1]
Derivative Liability, Fair Value, Gross Liability	6	[1]	0	[1]
Fuel derivatives | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	23	[1]	355	[1]
Derivative Liability, Fair Value, Gross Liability	7	[1]	16	[1]
Fuel derivatives | Designated as Hedging Instrument | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	19	[1]	0	[1]
Derivative Liability, Fair Value, Gross Liability	72	[1]	0	[1]
Fuel derivatives | Designated as Hedging Instrument | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	111	[1]	0	[1]
Derivative Liability, Fair Value, Gross Liability	30	[1]	0	[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	114	[1]	375	[1]
Derivative Liability, Fair Value, Gross Liability	95	[1]	327	[1]
Fuel derivatives | Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	24	[1]	233	[1]
Derivative Liability, Fair Value, Gross Liability	31	[1]	351	[1]
Fuel derivatives | Not Designated as Hedging Instrument | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	68	[1]	10	[1]
Derivative Liability, Fair Value, Gross Liability	106	[1]	60	[1]
Fuel derivatives | Not Designated as Hedging Instrument | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	38	[1]	0	[1]
Derivative Liability, Fair Value, Gross Liability	159	[1]	0	[1]
Interest rate derivatives | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Cash collateral deposits provided to counterparty - noncurrent	41		89
Interest rate derivatives | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	22	[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	$ 90	[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.