Financial Derivative Instruments Narrative (Details) - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Feb. 10, 2015	Dec. 31, 2006
Derivative [Line Items]
Deferred Gain (Loss) on Discontinuation of Interest Rate Fair Value Hedge						$ 12,000,000
Premiums paid for fuel derivative contracts		$ 22,000,000	$ 17,000,000	$ 48,000,000	$ 34,000,000
Current Unrealized Net Gains (Losses) in OCI		(323,000,000)		(323,000,000)
Maximum sum of derivatives of counterparty to be included in other (less than $20 million)		$ 10,000,000		$ 10,000,000
Cash collateral provided as a percentage of derivative contract value (in hundredths)		100.00%		100.00%
Letter of Credit Percent of Collateral		100.00%		100.00%
5.75% Notes due 2016
Derivative [Line Items]
Debt Instrument, Face Amount							$ 300,000,000
Stated interest rate							5.75%
Fuel derivatives
Derivative [Line Items]
GainLossOnDerivativeNetDueToLossOfHedgeAccounting		$ 7,000,000		$ 4,000,000
Cash Flow Hedging
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	[1]	(44,000,000)	(2,000,000)	(93,000,000)	(6,000,000)
Cash Flow Hedging | Fuel derivatives
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	[1],[2]	(41,000,000)	1,000,000	(87,000,000)	1,000,000
Cash Flow Hedging | Interest rate derivatives
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	[1],[2]	$ (3,000,000)	$ (3,000,000)	$ (6,000,000)	$ (7,000,000)
Remainder of 2015
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Percent of Required Need, Coverage		89.00%		89.00%

[1] Amounts related to fuel derivative contracts and interest rate derivatives are included in Fuel and oil and Interest expense, respectively.

[2] Net of tax