Fair Value Measurements Significant Unobservable Inputs (Details) (Significant Unobservable Inputs (Level 3) [Member], USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Option Contracts, Gas [Member] | Options Models, Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	42
Liabilities, Fair Value Disclosure	(18)
Option Contracts, Gas [Member] | Implied Volatilities [Member] | Maximum [Member] | Options Models, Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Expected Rates	39.00%
Option Contracts, Gas [Member] | Implied Volatilities [Member] | Minimum [Member] | Options Models, Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Expected Rates	1.00%
Option Contracts, Gas [Member] | Implied Correlations [Member] | Maximum [Member] | Options Models, Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Expected Rates	98.00%
Option Contracts, Gas [Member] | Implied Correlations [Member] | Minimum [Member] | Options Models, Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Expected Rates	12.00%
Full Requirements and Unit Contingent Contracts [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	213
Liabilities, Fair Value Disclosure	(34)
Full Requirements and Unit Contingent Contracts [Member] | Customer Migration Rate [Member] | Maximum [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Expected Rates	20.00%
Full Requirements and Unit Contingent Contracts [Member] | Customer Migration Rate [Member] | Minimum [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Expected Rates	0.00%
Full Requirements and Unit Contingent Contracts [Member] | Foward Price [Member] | Maximum [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Offered Quotes	122
Full Requirements and Unit Contingent Contracts [Member] | Foward Price [Member] | Minimum [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Offered Quotes	5
Option Contracts, Power [Member] | Options Models, Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	112
Liabilities, Fair Value Disclosure	(72)
Option Contracts, Power [Member] | Implied Volatilities [Member] | Maximum [Member] | Options Models, Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Expected Rates	274.00%
Option Contracts, Power [Member] | Implied Volatilities [Member] | Minimum [Member] | Options Models, Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Expected Rates	1.00%
Option Contracts, Power [Member] | Implied Correlations [Member] | Maximum [Member] | Options Models, Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Expected Rates	98.00%
Option Contracts, Power [Member] | Implied Correlations [Member] | Minimum [Member] | Options Models, Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Expected Rates	12.00%
Forward Contracts [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	399
Liabilities, Fair Value Disclosure	(88)
Forward Contracts [Member] | Foward Price [Member] | Maximum [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Offered Quotes	184
Forward Contracts [Member] | Foward Price [Member] | Minimum [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Offered Quotes	9