Derivative Instruments and Hedging Activities (Income Statement Impact of Derivative Instruments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Cash Flow Hedges | Operating revenues
Derivative [Line Items]
Ineffective portion recognized in earnings					$ 1		$ 2
Economic Hedges | Operating revenues
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	6		20		17		26
Economic Hedges | Fuel
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	(6)		(2)		(1)		4
Trading Activities | Operating revenues
Derivative [Line Items]
Effect of realized and unrealized gains (losses)	30		41		50		57
Commodity contracts
Derivative [Line Items]
Unrealized derivative gains (losses), after-tax recorded in accumulated other comprehensive income (loss)	12		5		12		5
Commodity contracts | Cash Flow Hedges
Derivative [Line Items]
Beginning of period derivative gains (losses)					35	[1]	43	[1]
Effective portion of changes in fair value					19	[1]	(6)	[1]
End of period derivative gains (losses)	21	[1]	8	[1]	21	[1]	8	[1]
Commodity contracts | Cash Flow Hedges | Operating revenues
Derivative [Line Items]
Reclassification to earnings					(33)	[1]	(29)	[1]
Interest rate contracts
Derivative [Line Items]
Unrealized derivative gains (losses), after-tax recorded in accumulated other comprehensive income (loss)	(69)		(14)		(69)		(14)
Interest rate contracts | Cash Flow Hedges
Derivative [Line Items]
Beginning of period derivative gains (losses)					(90)	[1]	(16)	[1]
Effective portion of changes in fair value					(24)	[1]	(7)	[1]
End of period derivative gains (losses)	(114)	[1]	(23)	[1]	(114)	[1]	(23)	[1]
Interest rate contracts | Cash Flow Hedges | Operating revenues
Derivative [Line Items]
Reclassification to earnings					$ 0	[1]	$ 0	[1]

[1] Unrealized derivative gains (losses) are before income taxes. The after-tax amounts recorded in accumulated other comprehensive loss at June 30, 2012 and 2011 for commodity and interest rate contracts were $12 million and $(69) million, and $5 million and $(14) million, respectively.