Fair Value Measurements (Significant Unobservable Inputs Used in Valuation of Contracts) (Details) - Fair Value, Inputs, Level 3 [Member] | 3 Months Ended |
Mar. 31, 2020USD ($) |
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% |