Fair Value Measurements, Significant Unobservable Inputs Used in Valuation of Contracts (Details) (Level 3 [Member], USD $) In Millions, except Per Share data, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Forward Contracts - Power [Member] | Discounted Cash Flow Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets	$ 366
Liabilities	95
Forward Contracts - Power [Member] | Discounted Cash Flow Member] | Foward Price [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (in dollars per unit)	$ 10
Forward Contracts - Power [Member] | Discounted Cash Flow Member] | Foward Price [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (in dollars per unit)	$ 141
Option Contracts - Power [Member] | Options Models [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets	179
Liabilities	200
Option Contracts - Power [Member] | Options Models [Member] | Implied Correlations [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (as a percentage)	12.00%
Option Contracts - Power [Member] | Options Models [Member] | Implied Correlations [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (as a percentage)	98.00%
Option Contracts - Power [Member] | Options Models [Member] | Implied Volatilities [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (as a percentage)	1.00%
Option Contracts - Power [Member] | Options Models [Member] | Implied Volatilities [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (as a percentage)	154.00%
Option Contracts - Gas [Member] | Options Models [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets	43
Liabilities	10
Option Contracts - Gas [Member] | Options Models [Member] | Implied Correlations [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (as a percentage)	12.00%
Option Contracts - Gas [Member] | Options Models [Member] | Implied Correlations [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (as a percentage)	98.00%
Option Contracts - Gas [Member] | Options Models [Member] | Implied Volatilities [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (as a percentage)	1.00%
Option Contracts - Gas [Member] | Options Models [Member] | Implied Volatilities [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (as a percentage)	56.00%
Full Requirements and Unit Contingent Contracts [Member] | Discounted Cash Flow Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets	211
Liabilities	$ 33
Full Requirements and Unit Contingent Contracts [Member] | Discounted Cash Flow Member] | Foward Price [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (in dollars per unit)	$ 5
Full Requirements and Unit Contingent Contracts [Member] | Discounted Cash Flow Member] | Foward Price [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (in dollars per unit)	$ 127
Full Requirements and Unit Contingent Contracts [Member] | Discounted Cash Flow Member] | Customer Migration Rate [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (as a percentage)	0.00%
Full Requirements and Unit Contingent Contracts [Member] | Discounted Cash Flow Member] | Customer Migration Rate [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (as a percentage)	20.00%