Financial Derivative Instruments - Fair Values by Balance Sheet Location (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 524	[1]	$ 1,004	[1]
Derivative Liability, Fair Value, Gross Liability	502	[1]	880	[1]
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	186	[1]	386	[1]
Derivative Liability, Fair Value, Gross Liability	103	[1]	142	[1]
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	338	[1]	618	[1]
Derivative Liability, Fair Value, Gross Liability	399	[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	14		54
Fuel derivatives | Accounts And Other Receivables
Derivatives, Fair Value [Line Items]
Receivable from third parties for settled fuel contracts - current	43		0
Fuel derivatives | Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	35	[1]	0	[1]
Derivative Liability, Fair Value, Gross Liability	14	[1]	0	[1]
Fuel derivatives | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	114	[1]	355	[1]
Derivative Liability, Fair Value, Gross Liability	2	[1]	16	[1]
Fuel derivatives | Designated as Hedging Instrument | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	15	[1]	0	[1]
Derivative Liability, Fair Value, Gross Liability	0	[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	268	[1]	375	[1]
Derivative Liability, Fair Value, Gross Liability	239	[1]	327	[1]
Fuel derivatives | Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	64	[1]	233	[1]
Derivative Liability, Fair Value, Gross Liability	118	[1]	351	[1]
Fuel derivatives | Not Designated as Hedging Instrument | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	2	[1]	10	[1]
Derivative Liability, Fair Value, Gross Liability	5	[1]	60	[1]
Fuel derivatives | Not Designated as Hedging Instrument | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	4	[1]	0	[1]
Derivative Liability, Fair Value, Gross Liability	37	[1]	0	[1]
Interest rate derivatives | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Cash collateral deposits provided to counterparty - noncurrent	33		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	$ 87	[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.