Fair Value Measurements (Significant Unobservable Inputs Used in Valuation of Contracts) (Details) - Fair Value, Inputs, Level 3 [Member]	3 Months Ended
	Mar. 31, 2020 USD ($)
Derivative Financial Instruments, Liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liabilities, Fair Value Disclosure	$ 485,000,000
Derivative Financial Instruments, Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Fair Value Disclosure	2,025,000,000
Forward Contracts - Power [Member] | Derivative Financial Instruments, Liabilities [Member] | Forward Contracts - Power [Member] | Discounted Cash Flow Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liabilities, Fair Value Disclosure	111,000,000
Forward Contracts - Power [Member] | Derivative Financial Instruments, Assets [Member] | Forward Contracts - Power [Member] | Discounted Cash Flow Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Fair Value Disclosure	864,000,000
Forward contracts - Gas [Member] | Derivative Financial Instruments, Liabilities [Member] | Forward contracts - Gas [Member] | Discounted Cash Flow Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liabilities, Fair Value Disclosure	22,000,000
Forward contracts - Gas [Member] | Derivative Financial Instruments, Assets [Member] | Forward contracts - Gas [Member] | Discounted Cash Flow Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Fair Value Disclosure	271,000,000
Forward contracts - Congestion [Member] | Derivative Financial Instruments, Liabilities [Member] | Forward contracts - Congestion [Member] | Discounted Cash Flow Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liabilities, Fair Value Disclosure	4,000,000
Forward contracts - Congestion [Member] | Derivative Financial Instruments, Assets [Member] | Forward contracts - Congestion [Member] | Discounted Cash Flow Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Fair Value Disclosure	21,000,000
Option Contracts, Power [Member] | Derivative Financial Instruments, Liabilities [Member] | Option Contracts, Power [Member] | Option Models [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liabilities, Fair Value Disclosure	10,000,000
Option Contracts, Power [Member] | Derivative Financial Instruments, Assets [Member] | Option Contracts, Power [Member] | Option Models [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Fair Value Disclosure	33,000,000
Option Contracts, Gas [Member] | Derivative Financial Instruments, Liabilities [Member] | Option Contracts, Primarily Gas [Member] | Option Models [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liabilities, Fair Value Disclosure	248,000,000
Option Contracts, Gas [Member] | Derivative Financial Instruments, Assets [Member] | Option Contracts, Primarily Gas [Member] | Option Models [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Fair Value Disclosure	238,000,000
Full Requirements and Unit Contingent Contracts [Member] | Derivative Financial Instruments, Liabilities [Member] | Full Requirements and Unit Contingent Contracts [Member] | Discounted Cash Flow Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liabilities, Fair Value Disclosure	69,000,000
Full Requirements and Unit Contingent Contracts [Member] | Derivative Financial Instruments, Assets [Member] | Full Requirements and Unit Contingent Contracts [Member] | Discounted Cash Flow Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Fair Value Disclosure	574,000,000
Forward contracts - Other [Member] [Member] | Derivative Financial Instruments, Liabilities [Member] | Forward contracts - Other [Member] [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Liabilities, Fair Value Disclosure	21,000,000
Forward contracts - Other [Member] [Member] | Derivative Financial Instruments, Assets [Member] | Forward contracts - Other [Member] [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets, Fair Value Disclosure	24,000,000
Forward Price [Member] | Forward Contracts - Power [Member] | Forward Contracts - Power [Member] | Discounted Cash Flow Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward price (in dollars per energy unit)	2
Forward Price [Member] | Forward Contracts - Power [Member] | Forward Contracts - Power [Member] | Discounted Cash Flow Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward price (in dollars per energy unit)	194
Forward Price [Member] | Forward Contracts - Power [Member] | Forward Contracts - Power [Member] | Discounted Cash Flow Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward price (in dollars per energy unit)	26
Forward Price [Member] | Forward contracts - Gas [Member] | Forward contracts - Gas [Member] | Discounted Cash Flow Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward price (in dollars per energy unit)	1
Forward Price [Member] | Forward contracts - Gas [Member] | Forward contracts - Gas [Member] | Discounted Cash Flow Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward price (in dollars per energy unit)	6
Forward Price [Member] | Forward contracts - Gas [Member] | Forward contracts - Gas [Member] | Discounted Cash Flow Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward price (in dollars per energy unit)	2
Forward Price [Member] | Forward contracts - Congestion [Member] | Forward contracts - Congestion [Member] | Discounted Cash Flow Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward price (in dollars per energy unit)	(13)
Forward Price [Member] | Forward contracts - Congestion [Member] | Forward contracts - Congestion [Member] | Discounted Cash Flow Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward price (in dollars per energy unit)	32
Forward Price [Member] | Forward contracts - Congestion [Member] | Forward contracts - Congestion [Member] | Discounted Cash Flow Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward price (in dollars per energy unit)	0
Forward Price [Member] | Full Requirements and Unit Contingent Contracts [Member] | Full Requirements and Unit Contingent Contracts [Member] | Discounted Cash Flow Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward price (in dollars per energy unit)	6
Forward Price [Member] | Full Requirements and Unit Contingent Contracts [Member] | Full Requirements and Unit Contingent Contracts [Member] | Discounted Cash Flow Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward price (in dollars per energy unit)	748
Forward Price [Member] | Full Requirements and Unit Contingent Contracts [Member] | Full Requirements and Unit Contingent Contracts [Member] | Discounted Cash Flow Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Forward price (in dollars per energy unit)	$ 46
Implied Correlations [Member] | Option Contracts, Power [Member] | Option Contracts, Power [Member] | Option Models [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Expected Rates	31.00%
Implied Correlations [Member] | Option Contracts, Power [Member] | Option Contracts, Power [Member] | Option Models [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Expected Rates	84.00%
Implied Correlations [Member] | Option Contracts, Power [Member] | Option Contracts, Power [Member] | Option Models [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Expected Rates	50.00%
Implied Correlations [Member] | Option Contracts, Gas [Member] | Option Contracts, Primarily Gas [Member] | Option Models [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Expected Rates	31.00%
Implied Correlations [Member] | Option Contracts, Gas [Member] | Option Contracts, Primarily Gas [Member] | Option Models [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Expected Rates	100.00%
Implied Correlations [Member] | Option Contracts, Gas [Member] | Option Contracts, Primarily Gas [Member] | Option Models [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Expected Rates	53.00%
Implied Volatilities [Member] | Option Contracts, Power [Member] | Option Contracts, Power [Member] | Option Models [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Expected Rates	13.00%
Implied Volatilities [Member] | Option Contracts, Power [Member] | Option Contracts, Power [Member] | Option Models [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Expected Rates	295.00%
Implied Volatilities [Member] | Option Contracts, Power [Member] | Option Contracts, Power [Member] | Option Models [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Expected Rates	45.00%
Implied Volatilities [Member] | Option Contracts, Gas [Member] | Option Contracts, Primarily Gas [Member] | Option Models [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Expected Rates	14.00%
Implied Volatilities [Member] | Option Contracts, Gas [Member] | Option Contracts, Primarily Gas [Member] | Option Models [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Expected Rates	278.00%
Implied Volatilities [Member] | Option Contracts, Gas [Member] | Option Contracts, Primarily Gas [Member] | Option Models [Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Expected Rates	36.00%
Customer Migration Rate [Member] | Full Requirements and Unit Contingent Contracts [Member] | Full Requirements and Unit Contingent Contracts [Member] | Discounted Cash Flow Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Expected Rates	0.00%
Customer Migration Rate [Member] | Full Requirements and Unit Contingent Contracts [Member] | Full Requirements and Unit Contingent Contracts [Member] | Discounted Cash Flow Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Expected Rates	13.00%
Customer Migration Rate [Member] | Full Requirements and Unit Contingent Contracts [Member] | Full Requirements and Unit Contingent Contracts [Member] | Discounted Cash Flow Member] | Weighted Average [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs, Expected Rates	0.00%