Fair Value Measurements (Significant Unobservable Inputs) (Details) - Significant Unobservable Inputs (Level 3) [Member] $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) $ / MMBTU $ / MWh $ / energy_unit
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 1,179
Liabilities, Fair Value Disclosure	540
Forward Contracts - Power [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	636
Liabilities, Fair Value Disclosure	$ 252
Forward Contracts - Power [Member] | Forward Price [Member] | Maximum [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Offered Quotes, Price Per Energy Unit | $ / MWh	113
Forward Contracts - Power [Member] | Forward Price [Member] | Minimum [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Offered Quotes, Price Per Energy Unit | $ / MWh	6
Forward contracts - Gas [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 24
Liabilities, Fair Value Disclosure	$ 25
Forward contracts - Gas [Member] | Forward Price [Member] | Maximum [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Offered Quotes, Price Per Energy Unit | $ / MMBTU	6
Forward contracts - Gas [Member] | Forward Price [Member] | Minimum [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Offered Quotes, Price Per Energy Unit | $ / MMBTU	1
Forward contracts - Other [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 16
Liabilities, Fair Value Disclosure	$ 6
Forward contracts - Other [Member] | Forward Price [Member] | Maximum [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Offered Quotes, Price Per Energy Unit | $ / energy_unit	55
Forward contracts - Other [Member] | Forward Price [Member] | Minimum [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Offered Quotes, Price Per Energy Unit | $ / energy_unit	(18)
Option Contracts, Power [Member] | Options Models, Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 68
Liabilities, Fair Value Disclosure	$ 58
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	99.00%
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	(5.00%)
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	308.00%
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%
Options - primarily gas [Member] | Options Models, Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 105
Liabilities, Fair Value Disclosure	$ 164
Options - primarily 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	99.00%
Options - primarily 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	(5.00%)
Options - primarily 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	195.00%
Options - primarily 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%
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	$ 330
Liabilities, Fair Value Disclosure	$ 35
Full Requirements and Unit Contingent Contracts [Member] | Forward Price [Member] | Maximum [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Offered Quotes, Price Per Energy Unit | $ / MWh	239
Full Requirements and Unit Contingent Contracts [Member] | Forward Price [Member] | Minimum [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Offered Quotes, Price Per Energy Unit | $ / MWh	(20)
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%
NEER [Member] | Interest Rate Swap [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Liabilities, Fair Value Disclosure	$ 101