Fair Value Measurements and Credit Concentration (Net Assets and Liabilities Measured on Recurring Basis Level 3 Significant Unobservable Inputs) (Details) (Level 3 [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012
Energy Commodities, Net [Member] | Retail Natural Gas Contracts [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair value of net asset and (liability)	29	[1]
Fair Value Inputs [Abstract]
Fair value scheduled to deliver within next 12 months	15
Energy Commodities, Net [Member] | Retail Natural Gas Contracts [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Observable wholesale prices used as proxy for retail delivery points	100.00%	[2]
Energy Commodities, Net [Member] | Retail Natural Gas Contracts [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Observable wholesale prices used as proxy for retail delivery points	20.00%	[2]
Energy Commodities, Net [Member] | Retail Natural Gas Contracts [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Observable wholesale prices used as proxy for retail delivery points	69.00%	[2]
Energy Commodities, Net [Member] | Electric Contracts [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair value of net asset and (liability)	(6)	[3]
Fair Value Inputs [Abstract]
Fair value scheduled to deliver within next 12 months	(3)
Energy Commodities, Net [Member] | Electric Contracts [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Basis price between delivery points	33.00%	[2]
Energy Commodities, Net [Member] | Electric Contracts [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Basis price between delivery points	28.00%	[2]
Energy Commodities, Net [Member] | Electric Contracts [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Basis price between delivery points	28.00%	[2]
Energy Commodities, Net [Member] | Other Contacts [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair value of net asset and (liability)	(4)	[4]
Energy Commodities, Net [Member] | Other Contacts [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Various	100.00%	[2]
Energy Commodities, Net [Member] | Other Contacts [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Various	22.00%	[2]
Energy Commodities, Net [Member] | Other Contacts [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Various	65.00%	[2]
Auction Rate Securities [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair value of net asset and (liability)	24	[5]
Fair Value Inputs [Abstract]
Weighted average contractual maturity of auction rate securites (in years)	24
Auction Rate Securities [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Modeled from SIFMA Index	90.00%	[2]
Auction Rate Securities [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Modeled from SIFMA Index	15.00%	[2]
Auction Rate Securities [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Modeled from SIFMA Index	66.00%	[2]
Cross Currency Swaps [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Fair value of net asset and (liability)	3	[6]
Cross Currency Swaps [Member] | Discounted Cash Flow [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Credit valuation adjustment	37.00%	[2]
Cross Currency Swaps [Member] | Discounted Cash Flow [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Credit valuation adjustment	23.00%	[2]
Cross Currency Swaps [Member] | Discounted Cash Flow [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Credit valuation adjustment	32.00%	[2]

[1] Retail natural gas contracts extend through 2017. $15 million of the fair value is scheduled to deliver within the next 12 months. As the price of natural gas increases/(decreases), the fair value of the contracts (decreases)/increases.

[2] For energy commodities and auction rate securities, the range and weighted average represent the percentage of fair value derived from the unobservable inputs. For cross-currency swaps, the range and weighted average represent the percentage decrease in fair value due to the unobservable inputs used in the model to calculate the credit valuation adjustment.

[3] Electric contracts extend through 2014. $(3) million of the fair value is scheduled to deliver within the next 12 months. As the price of electric increases/(decreases), the fair value of the contracts (decreases)/increases.

[4] Other includes FTR and capacity contracts. The models used to calculate the fair value of these contracts use historical settlement prices and extrapolation of observable forward curves. Increases/(decreases) in the historical settled prices or forward prices will (decrease)/increase the fair value.

[5] Auction rate securities have a weighted average contractual maturity of 24 years. The model used to calculate fair value incorporates significant assumptions, including the assumptions that the auctions will continue to fail and that the securities will be held to maturity. As the modeled forward rates of the SIFMA Index increase/(decrease), the fair value of the securities increases/(decreases).

[6] Cross-currency swaps extend through 2021. The credit valuation adjustment incorporates projected probabilities of default and estimated recovery rates. As the credit valuation adjustment increases/(decreases), the fair value of the swaps (decreases)/increases.