Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) (USD $) In Millions	Jun. 30, 2011		Dec. 31, 2010
Designated as Hedging Instrument [Member]
Derivative assets hedging instruments	$ 7		$ 5
Derivative liabilities as hedging instruments	7		1
Designated as Hedging Instrument [Member] | Power [Member] | Mark to Market Derivative Assets [Member]
Derivative assets hedging instruments	5		3
Designated as Hedging Instrument [Member] | Power [Member] | Other Assets [Member]
Derivative assets hedging instruments	2		2
Designated as Hedging Instrument [Member] | Power [Member] | Mark to Market Derivative Liabilities [Member]
Derivative liabilities as hedging instruments	3		1
Designated as Hedging Instrument [Member] | Power [Member] | Other Deferred Credits and Liabilities [Member]
Derivative liabilities as hedging instruments	4
Not Designated as Hedging Instrument [Member]
Derivative assets hedging instruments	263	[1]	169	[1]
Derivative liabilities as hedging instruments	210	[1]	252	[1]
Not Designated as Hedging Instrument [Member] | Heating Oil [Member] | Mark to Market Derivative Assets [Member]
Derivative assets hedging instruments	49	[1]	42	[1]
Not Designated as Hedging Instrument [Member] | Heating Oil [Member] | Other Assets [Member]
Derivative assets hedging instruments	23	[1]	22	[1]
Not Designated as Hedging Instrument [Member] | Heating Oil [Member] | Mark to Market Derivative Liabilities [Member]
Derivative liabilities as hedging instruments	4	[1]	12	[1]
Not Designated as Hedging Instrument [Member] | Heating Oil [Member] | Other Deferred Credits and Liabilities [Member]
Derivative liabilities as hedging instruments			1	[1]
Not Designated as Hedging Instrument [Member] | Natural Gas [Member] | Mark to Market Derivative Assets [Member]
Derivative assets hedging instruments	5	[1]	4	[1]
Not Designated as Hedging Instrument [Member] | Natural Gas [Member] | Other Assets [Member]
Derivative assets hedging instruments	1	[1]	1	[1]
Not Designated as Hedging Instrument [Member] | Natural Gas [Member] | Mark to Market Derivative Liabilities [Member]
Derivative liabilities as hedging instruments	75	[1]	87	[1]
Not Designated as Hedging Instrument [Member] | Natural Gas [Member] | Other Deferred Credits and Liabilities [Member]
Derivative liabilities as hedging instruments	63	[1]	84	[1]
Not Designated as Hedging Instrument [Member] | Power [Member] | Mark to Market Derivative Assets [Member]
Derivative assets hedging instruments	100	[1]	78	[1]
Not Designated as Hedging Instrument [Member] | Power [Member] | Other Assets [Member]
Derivative assets hedging instruments	85	[1]	20	[1]
Not Designated as Hedging Instrument [Member] | Power [Member] | Mark to Market Derivative Liabilities [Member]
Derivative liabilities as hedging instruments	52	[1]	61	[1]
Not Designated as Hedging Instrument [Member] | Power [Member] | Other Deferred Credits and Liabilities [Member]
Derivative liabilities as hedging instruments	14	[1]	7	[1]
Not Designated as Hedging Instrument [Member] | Uranium [Member] | Mark to Market Derivative Assets [Member]
Derivative assets hedging instruments			2	[1]
Not Designated as Hedging Instrument [Member] | Uranium [Member] | Mark to Market Derivative Liabilities [Member]
Derivative liabilities as hedging instruments	1	[1]
Not Designated as Hedging Instrument [Member] | Uranium [Member] | Other Deferred Credits and Liabilities [Member]
Derivative liabilities as hedging instruments	$ 1	[1]

[1] Includes derivatives subject to regulatory deferral.