Fair Value Measurements (Quantitative Information About Level 3 Fair Value Measurements) (Details) (Level 3, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	156
Fair Value, Liabilities	818
Electricity Options | Option Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	12
Fair Value, Liabilities	86
Electricity Options | Option Model | Gas Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	25.00%
Electricity Options | Option Model | Gas Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	48.00%
Electricity Options | Option Model | Gas Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	38.00%
Electricity Options | Option Model | Power Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	29.00%
Expected prices (in dollars per unit)	24.50
Electricity Options | Option Model | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	60.00%
Expected prices (in dollars per unit)	52.30
Electricity Options | Option Model | Power Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	43.00%
Expected prices (in dollars per unit)	35.40
Electricity Forwards | Discounted Cash Flow
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	37
Fair Value, Liabilities	56
Electricity Forwards | Discounted Cash Flow | Power Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	2.10
Electricity Forwards | Discounted Cash Flow | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	54.00
Electricity Forwards | Discounted Cash Flow | Power Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	30.96
Electricity Congestion Contracts | Market Simulation Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	101
Fair Value, Liabilities	0
Electricity Congestion Contracts | Market Simulation Model | Gas Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	3.79
Electricity Congestion Contracts | Market Simulation Model | Gas Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	9.32
Electricity Congestion Contracts | Market Simulation Model | Power Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	(46.19)
Electricity Congestion Contracts | Market Simulation Model | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	240.30
Electricity Congestion Contracts | Market Simulation Model | Load Forecast | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Fair value inputs, power units (in megawatts)	7,645
Electricity Congestion Contracts | Market Simulation Model | Load Forecast | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Fair value inputs, power units (in megawatts)	26,334
Electricity Congestion Contracts | Discounted Cash Flow
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	49
Fair Value, Liabilities	13
Electricity Congestion Contracts | Discounted Cash Flow | Congestion Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	(8.20)
Electricity Congestion Contracts | Discounted Cash Flow | Congestion Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	10.32
Electricity Congestion Contracts | Discounted Cash Flow | Congestion Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	0.21
Gas Options | Option Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	0
Fair Value, Liabilities	48
Gas Options | Option Model | Gas Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	26.00%
Gas Options | Option Model | Gas Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	48.00%
Gas Options | Option Model | Gas Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	41.00%
Tolling | Option Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	13
Fair Value, Liabilities	671
Tolling | Option Model | Gas Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	18.00%
Tolling | Option Model | Gas Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	48.00%
Tolling | Option Model | Gas Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	23.00%
Tolling | Option Model | Power Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	26.00%
Expected prices (in dollars per unit)	20.00
Tolling | Option Model | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	60.00%
Expected prices (in dollars per unit)	89.50
Tolling | Option Model | Power Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	30.00%
Expected prices (in dollars per unit)	53.40
Netting
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	(56)
Fair Value, Liabilities	(56)