Financial Derivative Instruments - Fair Values by Balance Sheet Location (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 237	[1]	$ 503	[1]
Derivative Liability, Fair Value, Gross Liability	231	[1]	380	[1]
Accrued Liabilities
Derivatives, Fair Value [Line Items]
Deferred premium gain for fuel contracts, current	8		0
Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Deferred premium receipt for fuel contracts, noncurrent	4		2
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	120	[1]	303	[1]
Derivative Liability, Fair Value, Gross Liability	127	[1]	78	[1]
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	117	[1]	200	[1]
Derivative Liability, Fair Value, Gross Liability	104	[1]	302	[1]
Fuel derivatives | Accounts And Other Receivables
Derivatives, Fair Value [Line Items]
Receivable from third parties for settled fuel contracts - current	19		57
Fuel derivatives | Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	7	[1]	74	[1]
Derivative Liability, Fair Value, Gross Liability	2	[1]	0	[1]
Fuel derivatives | Designated as Hedging Instrument | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	4	[1]	0	[1]
Derivative Liability, Fair Value, Gross Liability	31	[1]	0	[1]
Fuel derivatives | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	95	[1]	209	[1]
Derivative Liability, Fair Value, Gross Liability	33	[1]	1	[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	17	[1]	175	[1]
Derivative Liability, Fair Value, Gross Liability	10	[1]	182	[1]
Fuel derivatives | Not Designated as Hedging Instrument | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	36	[1]	9	[1]
Derivative Liability, Fair Value, Gross Liability	40	[1]	21	[1]
Fuel derivatives | Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	64	[1]	16	[1]
Derivative Liability, Fair Value, Gross Liability	54	[1]	99	[1]
Interest rate derivatives | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Cash collateral deposits provided to counterparty - noncurrent	24		32
Interest rate derivatives | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	14	[1]	20	[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	$ 61	[1]	$ 77	[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.