Financial Risk Management Activities (Schedule Of Derivative Instruments Not Designated As Hedging Instruments And Impact On Results Of Operations) (Detail) - Energy-Related Contracts [Member] - USD ($) $ in Millions |
3 Months Ended | |
|---|---|---|
Mar. 31, 2018 |
Mar. 31, 2017 |
|
| Derivative Instruments, Gain (Loss) [Line Items] | ||
| Pre-Tax Gain (Loss) Recognized in Income on Derivatives | $ 32 | $ 78 |
| Operating Revenues [Member] | ||
| Derivative Instruments, Gain (Loss) [Line Items] | ||
| Pre-Tax Gain (Loss) Recognized in Income on Derivatives | 40 | 78 |
| Energy Costs [Member] | ||
| Derivative Instruments, Gain (Loss) [Line Items] | ||
| Pre-Tax Gain (Loss) Recognized in Income on Derivatives | $ (8) | $ 0 |