Fair Value Measurements (Quantitative Information About Level 3 Fair Value Measurements) (Details) (Level 3, USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Quantitative Information About Level 3 Measurements
Fair Value, Assets	113
Fair Value, Liabilities	1,043
Electricity Options \| Option Model
Quantitative Information About Level 3 Measurements
Fair Value, Assets	23
Fair Value, Liabilities	26
Electricity Options \| Option Model \| Gas Prices \| Minimum
Quantitative Information About Level 3 Measurements
Expected volatility rate (as a percent)	23.00%
Electricity Options \| Option Model \| Gas Prices \| Maximum
Quantitative Information About Level 3 Measurements
Expected volatility rate (as a percent)	41.00%
Electricity Options \| Option Model \| Gas Prices \| Weighted Average
Quantitative Information About Level 3 Measurements
Expected volatility rate (as a percent)	31.00%
Electricity Options \| Option Model \| Power Prices \| Minimum
Quantitative Information About Level 3 Measurements
Expected volatility rate (as a percent)	28.00%
Expected prices (in dollars per unit)	38.10
Electricity Options \| Option Model \| Power Prices \| Maximum
Quantitative Information About Level 3 Measurements
Expected volatility rate (as a percent)	60.00%
Expected prices (in dollars per unit)	58.80
Electricity Options \| Option Model \| Power Prices \| Weighted Average
Quantitative Information About Level 3 Measurements
Expected volatility rate (as a percent)	39.00%
Expected prices (in dollars per unit)	45.30
Electricity Forwards \| Discounted Cash Flow
Quantitative Information About Level 3 Measurements
Fair Value, Assets	3
Fair Value, Liabilities	1
Electricity Forwards \| Discounted Cash Flow \| Power Prices \| Minimum
Quantitative Information About Level 3 Measurements
Expected prices (in dollars per unit)	22.10
Electricity Forwards \| Discounted Cash Flow \| Power Prices \| Maximum
Quantitative Information About Level 3 Measurements
Expected prices (in dollars per unit)	64.50
Electricity Forwards \| Discounted Cash Flow \| Power Prices \| Weighted Average
Quantitative Information About Level 3 Measurements
Expected prices (in dollars per unit)	32.50
Gas Options \| Option Model
Quantitative Information About Level 3 Measurements
Fair Value, Assets	0
Fair Value, Liabilities	6
Gas Options \| Option Model \| Gas Prices \| Minimum
Quantitative Information About Level 3 Measurements
Expected volatility rate (as a percent)	24.00%
Gas Options \| Option Model \| Gas Prices \| Maximum
Quantitative Information About Level 3 Measurements
Expected volatility rate (as a percent)	41.00%
Gas Options \| Option Model \| Gas Prices \| Weighted Average
Quantitative Information About Level 3 Measurements
Expected volatility rate (as a percent)	35.00%
CRRs \| Market Simulation Model
Quantitative Information About Level 3 Measurements
Fair Value, Assets	100
Fair Value, Liabilities	0
CRRs \| Market Simulation Model \| Gas Prices \| Minimum
Quantitative Information About Level 3 Measurements
Expected prices (in dollars per unit)	2.95
CRRs \| Market Simulation Model \| Gas Prices \| Maximum
Quantitative Information About Level 3 Measurements
Expected prices (in dollars per unit)	7.78
CRRs \| Market Simulation Model \| Power Prices \| Minimum
Quantitative Information About Level 3 Measurements
Expected prices (in dollars per unit)	(13.90)
CRRs \| Market Simulation Model \| Power Prices \| Maximum
Quantitative Information About Level 3 Measurements
Expected prices (in dollars per unit)	226.75
CRRs \| Market Simulation Model \| Load Forecast \| Minimum
Quantitative Information About Level 3 Measurements
Fair value inputs, power units (in megawatts)	7,597
CRRs \| Market Simulation Model \| Load Forecast \| Maximum
Quantitative Information About Level 3 Measurements
Fair value inputs, power units (in megawatts)	26,612
Tolling \| Option Model
Quantitative Information About Level 3 Measurements
Fair Value, Assets	5
Fair Value, Liabilities	1,028
Tolling \| Option Model \| Gas Prices \| Minimum
Quantitative Information About Level 3 Measurements
Expected volatility rate (as a percent)	17.00%
Tolling \| Option Model \| Gas Prices \| Maximum
Quantitative Information About Level 3 Measurements
Expected volatility rate (as a percent)	41.00%
Tolling \| Option Model \| Gas Prices \| Weighted Average
Quantitative Information About Level 3 Measurements
Expected volatility rate (as a percent)	21.00%
Tolling \| Option Model \| Power Prices \| Minimum
Quantitative Information About Level 3 Measurements
Expected volatility rate (as a percent)	26.00%
Expected prices (in dollars per unit)	33.20
Tolling \| Option Model \| Power Prices \| Maximum
Quantitative Information About Level 3 Measurements
Expected volatility rate (as a percent)	60.00%
Expected prices (in dollars per unit)	100.80
Tolling \| Option Model \| Power Prices \| Weighted Average
Quantitative Information About Level 3 Measurements
Expected volatility rate (as a percent)	28.00%
Expected prices (in dollars per unit)	56.10
Netting
Quantitative Information About Level 3 Measurements
Fair Value, Assets	(18)
Fair Value, Liabilities	(18)