Financial Derivative Instruments Narrative (Details) - USD ($)		3 Months Ended		6 Months Ended	9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Jun. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	Feb. 10, 2015	Dec. 31, 2006
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Percent of Required Need, Coverage		43.00%			43.00%
Deferred Gain (Loss) on Discontinuation of Interest Rate Fair Value Hedge							$ 12,000,000
Amount paid for purchases and sales of offsetting derivative instruments				$ 39,000,000
Hedge settlement losses recognized in Fuel and oil expense		$ 245,000,000
Premiums paid for fuel derivative contracts		33,000,000	$ 15,000,000		$ 81,000,000	$ 49,000,000
Current Unrealized Net Gains (Losses) in OCI		(471,000,000)			(471,000,000)
Maximum sum of derivatives of counterparty to be included in other (less than $20 million)		$ 30,000,000			$ 30,000,000
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%
Cash Flow Hedging
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	[1]	$ (82,000,000)	(8,000,000)		$ (175,000,000)	(13,000,000)
Cash Flow Hedging | Fuel derivatives
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	[1],[2]	(79,000,000)	(4,000,000)		(166,000,000)	(3,000,000)
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)	$ (4,000,000)		$ (9,000,000)	$ (10,000,000)
Fourth Quarter 2015 And First Half 2016
Derivative [Line Items]
Amount paid for purchases and sales of offsetting derivative instruments		$ 294,000,000

[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