Financial Derivative Instruments Narrative (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Percent of Required Need, Coverage		15.00%
Premiums paid for fuel derivative contracts		$ 35	$ 26
Current Unrealized Net Gains (Losses) in OCI		(547)
Maximum sum of derivatives of counterparty to be included in other (less than $20 million)		$ 5
Maximum cash collateral provided as a percentage of derivative contract value (in hundredths)		100.00%
Maximum letter of credit percent of collateral		100.00%
Cash Flow Hedging
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	[1]	$ (197)	(48)
Cash Flow Hedging | Fuel derivatives
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	[1],[2]	(195)	(45)
Cash Flow Hedging | Interest rate derivatives
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	[1],[2]	$ (2)	$ (3)
Remainder of Current Year
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Percent of Required Need, Coverage		60.00%

[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