Fair Value Measurements (Significant Unobservable Inputs) (Details) - Significant Unobservable Inputs (Level 3) [Member] $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / MMBTU $ / MWh $ / energy_unit
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 1,358
Liabilities, Fair Value Disclosure	660
Forward Contracts - Power [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	845
Liabilities, Fair Value Disclosure	$ 328
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	130
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	0
Forward contracts - Gas [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 26
Liabilities, Fair Value Disclosure	$ 13
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	7
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	2
Forward contracts - Other [Member] | Discounted Cash Flow Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 0
Liabilities, Fair Value Disclosure	$ 5
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	57
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	(40)
Option Contracts, Power [Member] | Options Models, Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 47
Liabilities, Fair Value Disclosure	$ 17
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	100.00%
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	1.00%
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	493.00%
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	8.00%
Options - primarily gas [Member] | Options Models, Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 165
Liabilities, Fair Value Disclosure	$ 199
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	100.00%
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	1.00%
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	290.00%
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	$ 275
Liabilities, Fair Value Disclosure	$ 98
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	293
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	(29)
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	$ 132