Fair Value Measurements (Quantitative Information About Level 3 Fair Value Measurements) (Details) (Level 3, USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	164
Fair Value, Liabilities	507
Electricity Options | Option Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	42
Fair Value, Liabilities	40
Electricity Options | Option Model | Gas Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	24.00%
Electricity Options | Option Model | Gas Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	57.00%
Electricity Options | Option Model | Gas Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	41.00%
Electricity Options | Option Model | Power Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	30.00%
Expected prices (in dollars per unit)	34.17
Electricity Options | Option Model | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	97.00%
Expected prices (in dollars per unit)	56.92
Electricity Options | Option Model | Power Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	54.00%
Expected prices (in dollars per unit)	42.70
Electricity Forwards | Discounted Cash Flow
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	21
Fair Value, Liabilities	31
Electricity Forwards | Discounted Cash Flow | Power Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	13.00
Electricity Forwards | Discounted Cash Flow | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	58.23
Electricity Forwards | Discounted Cash Flow | Power Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	32.61
Electricity Congestion Contracts | Market Simulation Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	92
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	73
Fair Value, Liabilities	24
Electricity Congestion Contracts | Discounted Cash Flow | Congestion Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	(15.52)
Electricity Congestion Contracts | Discounted Cash Flow | Congestion Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	23.03
Electricity Congestion Contracts | Discounted Cash Flow | Congestion Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected prices (in dollars per unit)	0.15
Gas Options | Option Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	0
Fair Value, Liabilities	28
Gas Options | Option Model | Gas Prices | Minimum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	27.00%
Gas Options | Option Model | Gas Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	57.00%
Gas Options | Option Model | Gas Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	47.00%
Tolling | Option Model
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	0
Fair Value, Liabilities	448
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)	57.00%
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)	29.89
Tolling | Option Model | Power Prices | Maximum
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	97.00%
Expected prices (in dollars per unit)	88.04
Tolling | Option Model | Power Prices | Weighted Average
Quantitative Information About Level 3 Measurements [Line Items]
Expected volatility rate (as a percent)	31.00%
Expected prices (in dollars per unit)	57.30
Netting
Quantitative Information About Level 3 Measurements [Line Items]
Fair Value, Assets	(64)
Fair Value, Liabilities	(64)