Financial Risk Management Activities (Schedule Of Reconciliation For Derivative Activity Included In Accumulated Other Comprehensive Loss) (Detail) - USD ($) $ in Millions |
3 Months Ended | 12 Months Ended | |
|---|---|---|---|
Mar. 31, 2018 |
Mar. 31, 2017 |
Dec. 31, 2017 |
|
| Derivative Instruments and Hedging Activities Disclosures [Line Items] | |||
| Gain (Loss) in AOCI | $ (16) | $ 30 | |
| (Gain) Loss into Income | 10 | (9) | |
| Cash Flow Hedges [Member] | |||
| Derivative Instruments and Hedging Activities Disclosures [Line Items] | |||
| Pre-Tax Balance at Beginning of Period | 0 | 3 | $ 3 |
| Gain (Loss) in AOCI | 0 | 0 | |
| (Gain) Loss into Income | 0 | (3) | |
| Pre-Tax Balance at End of Period | 0 | 0 | |
| After-Tax Balance at Beginning of Period | 0 | 2 | 2 |
| Gain (Loss) in AOCI | 0 | 0 | 0 |
| (Gain) Loss into Income | 0 | $ 0 | (2) |
| After-Tax Balance at End of Period | $ 0 | $ 0 | |