Fair Value Measurements Fair Value, Option, Quantitative Disclosures (Details) (USD $)	9 Months Ended
	Sep. 30, 2012
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets	$ 170,000,000
Fair Value of Derivative Liabilities	67,000,000
Natural Gas | Physical Financial Fowards Futures
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets	29,000,000	[1]
Fair Value of Derivative Liabilities	19,000,000	[1]
Natural Gas | Physical Financial Options
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets	4,000,000
Fair Value of Derivative Liabilities	18,000,000	[1]
Electricity | Physical Financial Fowards Futures
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets	63,000,000
Fair Value of Derivative Liabilities	13,000,000
Electricity | Full Requirements Contracts
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets	31,000,000
FTRs | Physical Financial Fowards Futures
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets	6,000,000
Fair Value of Derivative Liabilities	3,000,000
Capacity | Physical Financial Fowards Futures
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets	8,000,000
Liquids | Physical Financial Fowards Futures
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value of Derivative Assets	29,000,000
Fair Value of Derivative Liabilities	14,000,000
Fair Value, Option, Quantitative Disclosures, Assets | Natural Gas | Physical Financial Fowards Futures | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	(1)	[1],[2]
Fair Value, Option, Quantitative Disclosures, Assets | Natural Gas | Physical Financial Fowards Futures | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	6	[1],[2]
Fair Value, Option, Quantitative Disclosures, Assets | Natural Gas | Physical Financial Fowards Futures | Wieghted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	3	[1],[2]
Fair Value, Option, Quantitative Disclosures, Assets | Natural Gas | Physical Financial Options | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	0	[2]
Price Volatility (percentage)	20.00%	[2],[3]
Price Correlation (percentage)	73.00%	[2],[4]
Fair Value, Option, Quantitative Disclosures, Assets | Natural Gas | Physical Financial Options | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	5	[2]
Price Volatility (percentage)	53.00%	[2],[3]
Price Correlation (percentage)	99.00%	[2],[4]
Fair Value, Option, Quantitative Disclosures, Assets | Natural Gas | Physical Financial Options | Wieghted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	4	[2]
Price Volatility (percentage)	25.00%	[2],[3]
Price Correlation (percentage)	77.00%	[2],[4]
Fair Value, Option, Quantitative Disclosures, Assets | Electricity | Physical Financial Fowards Futures | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	30	[2]
Fair Value, Option, Quantitative Disclosures, Assets | Electricity | Physical Financial Fowards Futures | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	64	[2]
Fair Value, Option, Quantitative Disclosures, Assets | Electricity | Physical Financial Fowards Futures | Wieghted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	44	[2]
Fair Value, Option, Quantitative Disclosures, Assets | Electricity | Full Requirements Contracts | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	90
Load Shaping	2.00%	[2],[5]
Usage Factor	1.00%	[2],[6]
Fair Value, Option, Quantitative Disclosures, Assets | Electricity | Full Requirements Contracts | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	420
Load Shaping	6.00%	[2],[5]
Usage Factor	15.00%	[2],[6]
Fair Value, Option, Quantitative Disclosures, Assets | Electricity | Full Requirements Contracts | Wieghted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	40	[2]
Load Shaping	4.00%	[2],[5]
Usage Factor	8.00%	[2],[6]
Fair Value, Option, Quantitative Disclosures, Assets | FTRs | Physical Financial Fowards Futures | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	(5)	[2]
Fair Value, Option, Quantitative Disclosures, Assets | FTRs | Physical Financial Fowards Futures | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	7	[2]
Fair Value, Option, Quantitative Disclosures, Assets | FTRs | Physical Financial Fowards Futures | Wieghted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	0	[2]
Fair Value, Option, Quantitative Disclosures, Assets | Capacity | Physical Financial Fowards Futures | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	95	[2]
Fair Value, Option, Quantitative Disclosures, Assets | Capacity | Physical Financial Fowards Futures | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	115	[2]
Fair Value, Option, Quantitative Disclosures, Assets | Capacity | Physical Financial Fowards Futures | Wieghted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	100	[2]
Fair Value, Option, Quantitative Disclosures, Assets | Liquids | Physical Financial Fowards Futures | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	0	[2]
Fair Value, Option, Quantitative Disclosures, Assets | Liquids | Physical Financial Fowards Futures | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	3	[2]
Fair Value, Option, Quantitative Disclosures, Assets | Liquids | Physical Financial Fowards Futures | Wieghted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	1	[2]
Fair Value, Option, Quantitative Disclosures, Liabilities | Natural Gas | Physical Financial Fowards Futures | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	(1)	[1],[2]
Fair Value, Option, Quantitative Disclosures, Liabilities | Natural Gas | Physical Financial Fowards Futures | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	6	[1],[2]
Fair Value, Option, Quantitative Disclosures, Liabilities | Natural Gas | Physical Financial Fowards Futures | Wieghted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	4	[1],[2]
Fair Value, Option, Quantitative Disclosures, Liabilities | Natural Gas | Physical Financial Options | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	3	[1],[2]
Price Volatility (percentage)	20.00%	[2],[3]
Price Correlation (percentage)	99.00%	[2],[4]
Fair Value, Option, Quantitative Disclosures, Liabilities | Natural Gas | Physical Financial Options | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	5	[1],[2]
Price Volatility (percentage)	50.00%	[2],[3]
Price Correlation (percentage)	99.00%	[2],[4]
Fair Value, Option, Quantitative Disclosures, Liabilities | Natural Gas | Physical Financial Options | Wieghted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	4	[1],[2]
Price Volatility (percentage)	27.00%	[2],[3]
Price Correlation (percentage)	99.00%	[2],[4]
Fair Value, Option, Quantitative Disclosures, Liabilities | Electricity | Physical Financial Fowards Futures | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	27	[2]
Fair Value, Option, Quantitative Disclosures, Liabilities | Electricity | Physical Financial Fowards Futures | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	64	[2]
Fair Value, Option, Quantitative Disclosures, Liabilities | Electricity | Physical Financial Fowards Futures | Wieghted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	43	[2]
Fair Value, Option, Quantitative Disclosures, Liabilities | FTRs | Physical Financial Fowards Futures | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	(2)	[2]
Fair Value, Option, Quantitative Disclosures, Liabilities | FTRs | Physical Financial Fowards Futures | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	10	[2]
Fair Value, Option, Quantitative Disclosures, Liabilities | FTRs | Physical Financial Fowards Futures | Wieghted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	1	[2]
Fair Value, Option, Quantitative Disclosures, Liabilities | Liquids | Physical Financial Fowards Futures | Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	1	[2],[7]
Fair Value, Option, Quantitative Disclosures, Liabilities | Liquids | Physical Financial Fowards Futures | Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	3	[2],[7]
Fair Value, Option, Quantitative Disclosures, Liabilities | Liquids | Physical Financial Fowards Futures | Wieghted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Market Price (per unit)	$ 2	[2]

[1] Includes basis.

[2] Averages weighted by volume.

[3] Represents volatilities unrepresented in published markets.

[4] Represents intra-price correlations for which markets do not exist.

[5] Converts block monthly loads to 24-hour load shapes.

[6] Represents expected increase (decrease) in sales volumes compared to historical usage.

[7] Includes NGLs and oil.