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	6 Months Ended
	Jun. 30, 2013
Forward Contracts [Member] | Discounted Cash Flow Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets	$ 380
Liabilities	85
Forward 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)	$ 9
Forward 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)	$ 196
Interest rate swaps [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets	91
Forward contracts - Gas [Member] | Discounted Cash Flow Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets	25
Liabilities	16
Forward contracts - Gas [Member] | Discounted Cash Flow Member] | Foward Price [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (in dollars per unit)	$ 3
Forward contracts - Gas [Member] | Discounted Cash Flow Member] | Foward Price [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (in dollars per unit)	$ 7
Forward contracts - Other [Member] [Member] | Discounted Cash Flow Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets	11
Liabilities	7
Forward contracts - Other [Member] [Member] | Discounted Cash Flow Member] | Foward Price [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (in dollars per unit)	$ 1
Forward contracts - Other [Member] [Member] | Discounted Cash Flow Member] | Foward Price [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected rates (in dollars per unit)	$ 245
Option Contracts - Power [Member] | Options Models [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets	105
Liabilities	130
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)	156.00%
Option Contracts - Gas [Member] | Options Models [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets	26
Liabilities	24
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)	33.00%
Full Requirements and Unit Contingent Contracts [Member] | Discounted Cash Flow Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets	215
Liabilities	$ 25
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)	$ (4)
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)	$ 152
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%