Derivative Instruments (Income Statement Disclosure) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Effect of Cash Flow Hedges on Results of Operations [Abstract]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	$ (25)		$ (45)		$ 128		$ (120)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(14)	[1]	(10)	[1]	(75)	[1]	(42)	[1]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	135		(213)		78		125
Gains (losses) on commodity contracts, recorded as regulatory assets and or liabilities on the balance sheet due to regulatory treatment	(22)		90		(27)		(86)
Commodity contracts [Member]
Effect of Cash Flow Hedges on Results of Operations [Abstract]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net			0				0
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net			2				6
Commodity contracts [Member] | Gains (losses) included in operating revenues [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	138		(218)		111		102
Commodity contracts [Member] | Gains (losses) included in fuel, purchased power and interchange [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(9)		(4)		2		36
Interest Rate Contracts [Member]
Effect of Cash Flow Hedges on Results of Operations [Abstract]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	(29)		(39)		136		(104)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(15)		(14)		(45)		(44)
Interest Rate Contracts [Member] | Gain Loss Included In Interest Expense Member
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	3		0		14		0
Foreign currency swaps [Member]
Effect of Cash Flow Hedges on Results of Operations [Abstract]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	4		(6)		(8)		(16)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	1	[2]	2	[2]	(30)	[3]	(4)	[4]
Foreign currency swaps [Member] | Gains (losses) included in Other - net [Member]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	3		9		(49)		(13)
Fair Value Hedging [Member]
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net [Abstract]
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net	2		6		(55)		41
Cash Flow Hedging [Member] | Currency Swap [Member] | Gains (losses) included in interest expense [Member]
Derivative [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ (1)		$ (1)		$ (3)		$ (2)

[1] Included in operating revenues for commodity contracts and interest expense for interest rate contracts.

[2] Loss of approximately $1 million is included in interest expense and the balance is included in other - net.

[3] Loss of approximately $3 million is included in interest expense and the balance is included in other - net.

[4] Loss of approximately $2 million is included in interest expense and the balance is included in other - net.