Financial Derivative Instruments Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Percent of Required Need, Coverage	66.00%
Premiums paid for fuel derivative contracts	$ 34	$ 35
Current Unrealized Net Gains (Losses) in OCI	(275)
Maximum sum of derivatives of counterparty to be included in other (less than $5 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%
Remainder of Current Year
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Percent of Required Need, Coverage	61.00%