Financial Derivative Instruments - Fair Values by Balance Sheet Location (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 769	[1]	$ 503	[1]
Derivative Liability, Fair Value, Gross Liability	399	[1]	380	[1]
Deferred premium receipt for fuel contracts, noncurrent	12		2
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	527	[1]	303	[1]
Derivative Liability, Fair Value, Gross Liability	76	[1]	78	[1]
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	242	[1]	200	[1]
Derivative Liability, Fair Value, Gross Liability	323	[1]	302	[1]
Fuel derivatives | Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Total Collateral Already Received Aggregate Fair Value	23		0
Fuel derivatives | Accounts Payable Balance Sheet Location [Member]
Derivatives, Fair Value [Line Items]
Due To Third Parties For Settled Fuel Contracts	2		0
Fuel derivatives | Accounts And Other Receivables
Derivatives, Fair Value [Line Items]
Receivable from third parties for settled fuel contracts - current	34		57
Fuel derivatives | Other Assets
Derivatives, Fair Value [Line Items]
Total Collateral Already Received Aggregate Fair Value	82		0
Fuel derivatives | Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	125	[1]	74	[1]
Derivative Liability, Fair Value, Gross Liability	1	[1]	0	[1]
Fuel derivatives | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	385	[1]	209	[1]
Derivative Liability, Fair Value, Gross Liability	3	[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	181	[1]	175	[1]
Derivative Liability, Fair Value, Gross Liability	216	[1]	182	[1]
Fuel derivatives | Not Designated as Hedging Instrument | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	[1]	9	[1]
Derivative Liability, Fair Value, Gross Liability	0	[1]	21	[1]
Fuel derivatives | Not Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	61	[1]	16	[1]
Derivative Liability, Fair Value, Gross Liability	107	[1]	99	[1]
Interest rate derivatives | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Cash collateral deposits provided to counterparty - noncurrent	32		32
Interest rate derivatives | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	17	[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	$ 72	[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.