Fair Value Measurements (Significant Unobservable Inputs Used in Valuation of Contracts) (Details) - Fair Value, Inputs, Level 3 [Member] | 6 Months Ended |
Jun. 30, 2020USD ($) |
Derivative Financial Instruments, Liabilities [Member] | |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] | |
Liabilities, Fair Value Disclosure | $ 432,000,000 |
Derivative Financial Instruments, Assets [Member] | |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] | |
Assets, Fair Value Disclosure | 1,758,000,000 |
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 | 102,000,000 |
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 | 793,000,000 |
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 | 24,000,000 |
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 | 263,000,000 |
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 | 6,000,000 |
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 | 22,000,000 |
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 | 25,000,000 |
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 | 45,000,000 |
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 | 196,000,000 |
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 | 168,000,000 |
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 | 62,000,000 |
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 | 436,000,000 |
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 | 17,000,000 |
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 | 31,000,000 |
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) | 215 |
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) | 28 |
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) | (6) |
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) | 30 |
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) | 790 |
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) | $ 47 |
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 | 40.00% |
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 | 85.00% |
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 | 55.00% |
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 | 40.00% |
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 | 59.00% |
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 | 17.00% |
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 | 510.00% |
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 | 62.00% |
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 | 16.00% |
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 | 135.00% |
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 | 38.00% |
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 | 122.00% |
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 | 2.00% |