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Fair Value Measurements (Significant Unobservable Inputs Used in Valuation of Contracts) (Details) - Fair Value, Inputs, Level 3 [Member] $ in Millions | 9 Months Ended
--- | ---
Sep. 30, 2018 USD ($) $ / MMBTU $ / MWh
Derivative Financial Instruments, Liabilities [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Liabilities, Fair Value Disclosure | $ 557
Derivative Financial Instruments, Assets [Member] |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |
Assets, Fair Value Disclosure | 1,227
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 | 265
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 | 813
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 | 13
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 | 26
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 | 21
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 | 51
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 | 153
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 | 143
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 | 105
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 | $ 194
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) | $ / MWh | (27)
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) | $ / MWh | 161
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) | $ / MMBTU | 2
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) | $ / MMBTU | 7
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) | $ / MWh | (17)
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) | $ / MWh | 561
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 | 1.00%
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 | 100.00%
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 | 1.00%
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 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 | 7.00%
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 | 197.00%
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 | 1.00%
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 | 110.00%
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 | 20.00%

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