Fair Value Measurements (Fair Value, Option, Quantitative Disclosures) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	$ 84,000,000
Derivative liabilities	59,000,000
Gas | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	13,000,000	[1]
Derivative liabilities	18,000,000	[1]
Gas | Physical and Financial Options:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	5,000,000
Derivative liabilities	12,000,000	[1]
Electricity | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	6,000,000
Derivative liabilities	1,000,000
Electricity | Full Requirements Contracts:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	27,000,000
FTRs | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	5,000,000
Derivative liabilities	3,000,000
Capacity | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	7,000,000
Liquids | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	21,000,000	[2]
Derivative liabilities	25,000,000	[2]
Fair Value, Option, Quantitative Disclosures, Assets | Minimum | Gas | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	(1)	[3]
Fair Value, Option, Quantitative Disclosures, Assets | Minimum | Gas | Physical and Financial Options:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	3	[3]
Price Volatility	21.00%	[4]
Price Correlation	73.00%	[5]
Fair Value, Option, Quantitative Disclosures, Assets | Minimum | Electricity | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	30	[3]
Fair Value, Option, Quantitative Disclosures, Assets | Minimum | Electricity | Full Requirements Contracts:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	8	[3]
Load Shaping	0.00%	[6]
Usage Factor	2.00%	[7]
Fair Value, Option, Quantitative Disclosures, Assets | Minimum | FTRs | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	(6)	[3]
Fair Value, Option, Quantitative Disclosures, Assets | Minimum | Capacity | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	95	[3]
Fair Value, Option, Quantitative Disclosures, Assets | Minimum | Liquids | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	0	[3]
Fair Value, Option, Quantitative Disclosures, Assets | Weighted Average | Gas | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	3	[8]
Fair Value, Option, Quantitative Disclosures, Assets | Weighted Average | Gas | Physical and Financial Options:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	4	[8]
Price Volatility	24.00%	[8]
Price Correlation	73.00%	[8]
Fair Value, Option, Quantitative Disclosures, Assets | Weighted Average | Electricity | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	50	[8]
Fair Value, Option, Quantitative Disclosures, Assets | Weighted Average | Electricity | Full Requirements Contracts:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	40	[8]
Load Shaping	5.00%	[8]
Usage Factor	8.00%	[8]
Fair Value, Option, Quantitative Disclosures, Assets | Weighted Average | FTRs | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	1	[8]
Fair Value, Option, Quantitative Disclosures, Assets | Weighted Average | Capacity | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	101	[8]
Fair Value, Option, Quantitative Disclosures, Assets | Weighted Average | Liquids | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	1	[8]
Fair Value, Option, Quantitative Disclosures, Assets | Maximum | Gas | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	6	[3]
Fair Value, Option, Quantitative Disclosures, Assets | Maximum | Gas | Physical and Financial Options:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	5	[3]
Price Volatility	36.00%	[4]
Price Correlation	73.00%	[5]
Fair Value, Option, Quantitative Disclosures, Assets | Maximum | Electricity | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	85	[3]
Fair Value, Option, Quantitative Disclosures, Assets | Maximum | Electricity | Full Requirements Contracts:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	439	[3]
Load Shaping	10.00%	[6]
Usage Factor	16.00%	[7]
Fair Value, Option, Quantitative Disclosures, Assets | Maximum | FTRs | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	7	[3]
Fair Value, Option, Quantitative Disclosures, Assets | Maximum | Capacity | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	115	[3]
Fair Value, Option, Quantitative Disclosures, Assets | Maximum | Liquids | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	3	[3]
Fair Value, Option, Quantitative Disclosures, Liabilities | Minimum | Gas | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	(1)	[3]
Fair Value, Option, Quantitative Disclosures, Liabilities | Minimum | Gas | Physical and Financial Options:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	3	[3]
Price Volatility	21.00%	[4]
Price Correlation	99.00%	[5]
Fair Value, Option, Quantitative Disclosures, Liabilities | Minimum | Electricity | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	25	[3]
Fair Value, Option, Quantitative Disclosures, Liabilities | Minimum | FTRs | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	(1)	[3]
Fair Value, Option, Quantitative Disclosures, Liabilities | Minimum | Liquids | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	1	[3]
Fair Value, Option, Quantitative Disclosures, Liabilities | Weighted Average | Gas | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	3	[8]
Fair Value, Option, Quantitative Disclosures, Liabilities | Weighted Average | Gas | Physical and Financial Options:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	5	[8]
Price Volatility	32.00%	[8]
Price Correlation	99.00%	[8]
Fair Value, Option, Quantitative Disclosures, Liabilities | Weighted Average | Electricity | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	39	[8]
Fair Value, Option, Quantitative Disclosures, Liabilities | Weighted Average | FTRs | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	0	[8]
Fair Value, Option, Quantitative Disclosures, Liabilities | Weighted Average | Liquids | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	2	[8]
Fair Value, Option, Quantitative Disclosures, Liabilities | Maximum | Gas | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	18	[3]
Fair Value, Option, Quantitative Disclosures, Liabilities | Maximum | Gas | Physical and Financial Options:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	8	[3]
Price Volatility	36.00%	[4]
Price Correlation	99.00%	[5]
Fair Value, Option, Quantitative Disclosures, Liabilities | Maximum | Electricity | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	65	[3]
Fair Value, Option, Quantitative Disclosures, Liabilities | Maximum | FTRs | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	18	[3]
Fair Value, Option, Quantitative Disclosures, Liabilities | Maximum | Liquids | Physical and Financial Forwards and Futures:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market Price	$ 3	[3]

[1] Includes basis.

[2] Includes NGLs

[3] Represents market prices beyond defined terms for Levels 1 & 2.

[4] Represents volatilities unrepresented in published markets.

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

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

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

[8] Averages weighted by volume.