Financial Derivative Instruments Narrative (Details) (USD $)	3 Months Ended				12 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014	Dec. 31, 2006
Derivative [Line Items]
Deferred Gain (Loss) on Discontinuation of Interest Rate Fair Value Hedge	$ 12,000,000
Amount paid for purchases and sales of offsetting derivative instruments					217,000,000
Premiums paid for fuel derivative contracts	26,000,000		17,000,000
Current Unrealized Net Gains (Losses) in OCI	(291,000,000)
Maximum sum of derivatives of counterparty to be included in other (less than $20 million)	20,000,000
Cash collateral provided as a percentage of derivative contract value (in hundredths)	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	11,000,000
Cash Flow Hedging
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(48,000,000)	[1]	(4,000,000)	[1]
Cash Flow Hedging | Fuel derivatives
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(45,000,000)	[1],[2]	0	[1],[2]
Cash Flow Hedging | Interest rate derivatives
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ (3,000,000)	[1],[2]	$ (4,000,000)	[1],[2]

[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