Derivative Instruments (Income Statement Disclosure) (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
Effect of Cash Flow Hedges on Results of Operations [Abstract]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	$ (66)		$ (109)		$ (75)		$ (124)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(7)		(3)		(32)		(28)
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net [Abstract]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	196		183		338		1
Gains (losses) on commodity contracts, recorded as regulatory assets and or liabilities on the balance sheet due to regulatory treatment	76		(68)		(176)		(68)
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		0		0
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	3		14		4		19
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	130		154		320		2
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	51		23		40		(2)
Interest Rate Contract [Member]
Effect of Cash Flow Hedges on Results of Operations [Abstract]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	(66)		(119)		(65)		(118)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(14)		(24)		(30)		(43)
Interest Rate Contract [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	0		4		0		4
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	0		10		(10)		(6)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	4	[1]	7	[1]	(6)	[2]	(4)	[3]
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	15		2		(22)		(3)
Fair Value Hedging [Member]
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net [Abstract]
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net	34		9		35		3
Number of instruments held	6		3		6		3
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)		$ (2)		$ (3)

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

[2] Loss of approximately $2 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.